How do I find out what the fuel/gas capacity is?

Your friend's husband may also know if there is a switch in the dash area which would allow you to read the level of the second tank on your fuel gauge.

I would think there would be another fill neck for that tank, maybe get underneath with a mirror and a light see if you can look at the top of the Transfer Flow tank for a filler pipe and wires to it. That will tell you if there is another way to fill it and if there is a sending unit for the fuel gauge.
I love my extra tank gives me the advantage of fueling when I want to.
let us know what you find out, good luck.

fltraveler said:

My husband and I have both been looking at their website. We'll have to go do some investigating for another fill tube.

What do you mean a value to separate the tanks??

And yes, it does look like it says 63 gallons. So in theory the original tank should be 36 gallons according to my Ford Econoline manual. And apparently the add on tank is 63 gallons.

On our trip bringing it home the most my husband filled it was 41 gallons. Our fuel gage said it had just a bit less than a quarter of a tank when we stopped and filled it 41 gallons.

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Most installations have a valve to separate the tanks. you run on one tank till it is empty then switch to the outer tank. There should be a switch on the dash that will let you see the level in the auxiliary tank. look  for a switch on the floor under the front of the seat that may be to a valve.
If there isn't a valve when your husband filled it the level would have dropped quickly as the fuel backfilled the other tank. It would have looked like the worst milage or a major leak. ;D
Bill

I can't find any way at all to fill just that extra tank. I feel like I've looked everywhere.

Here is a photo of the other side of the tank. Maybe it looks like there is a tube going into it on the top?

Also is a picture of underneath the steering wheel. I still see nothing there or on the dash that would indicate anything at all.

There is an approved installer for Transfer Flow Inc in Orlando. Maybe we need to take it to them to see if they can figure it out? Or wait and find our if it looks like someone installed themselves in a way that doesn't even work right??

That is a wild set up, I would definitely have Them (approved installer) Look at it and maybe communicate with the company directly. If possible remove one tank, that is a lot of fuel to be carrying around and dragging low in the rear IMO.

fltraveler said:

I can't find any way at all to fill just that extra tank. I feel like I've looked everywhere.

Here is a photo of the other side of the tank. Maybe it looks like there is a tube going into it on the top?

Also is a picture of underneath the steering wheel. I still see nothing there or on the dash that would indicate anything at all.

There is an approved installer for Transfer Flow Inc in Orlando. Maybe we need to take it to them to see if they can figure it out? Or wait and find our if it looks like someone installed themselves in a way that doesn't even work right??

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Look for any switch that doesn't show up in the owner's manual or dosen't seem to do anything when moved. Look at the floor by the seat. It would need to be where you can reach it when driving.
Taking it in is always an option. If it isn't installed right maybe they can fix it so it works.
Bill

WILDEBILL308 said:

Look for any switch that doesn't show up in the owner's manual or dosen't seem to do anything when moved. Look at the floor by the seat. It would need to be where you can reach it when driving.
Taking it in is always an option. If it isn't installed right maybe they can fix it so it works.
Bill

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Bill I'm a little confused which doesn't take much. Why are we looking for a switch? What would a switch do?  The picture shows that both tanks are connected to each other by a common hose. There is no valve in the line so that means that as one tank is being filled, gas would flow over into the other tank through the line. And when the fuel is being used, both tanks would drain evenly. Am I missing something?

The original tank is 36 gallons. The add on is 63. So if fuel flows into the original there's just no way it would ever fill the add on tank...UNLESS they rerouted the fill tube to fill the add on tank first?? Maybe that's what they did??

fltraveler said:

The original tank is 36 gallons. The add on is 63. So if fuel flows into the original there's just no way it would ever fill the add on tank...UNLESS they rerouted the fill tube to fill the add on tank first?? Maybe that's what they did??

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No matter which tank is being filled first, fuel will flow from one tank to the other through the line at the bottom of the tanks. You show that line in one of the pictures. The only way it would not flow would be if you added a valve in that hose between both tanks.

If the tops of the tanks are up tight to the floor board, there's no way you could fill into one and have the other overflow. If you have a fuel gauge, when it says it's 1/2 full, the level in both tanks would be 1/2 full.  Which would mean you have approximately 50 gallons left.  You only need one gauge.
When filling, it will take some time for the fuel to flow over to the other tank because of the small size of the cross connect hose.  You'll have to fill, take a break, fill some more and take another break and fill more. Eventually, both tanks will be full. 

Rene T said:

Bill I'm a little confused which doesn't take much. Why are we looking for a switch? What would a switch do?  The picture shows that both tanks are connected to each other by a common hose. There is no valve in the line so that means that as one tank is being filled, gas would flow over into the other tank through the line. And when the fuel is being used, both tanks would drain evenly. Am I missing something?

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I agree.

If I'm understanding what I'm seeing.. You simply have a Gravity fed Auxiliary fuel tank.

When you fill the main (Main OEM) tank, the fuel will Slooowly, via gravity, flow into the Aux tank, until the two tanks are full. The Aux tank won't overfill the main tank because when the main tank is full.. the fuel nozzle will cut off. Full being when both tanks are ONLY as Full as the main tanks volume highth. (Liquids will seek their own level ?)

But.. It's going to be a Slooow process because the Gas Station nozzle will flow at a faster rate (GPM) than that fuel line between the two tanks.. can deliver (GPM) fuel to the Aux tank.

If you fill up with the Gas Station nozzle set at a Very Slow Fill rate.. it might smooth out the process ?

If all of this is fact.. It's going to Be: Fill, Click, wait, check the gauge.. fill, Click, wait, check the gauge.

You will have Gravity on your side though, as the main tank fills, because the weight of the maint tanks fuel will put a little force on the flow to the Aux tank.

As to what this does to the Fuel Gauge ? No Clue. Unless you will see the gauge move slower from Full, toward Empty, due to the extra fuel in the aux tank.

On Edit: Looks like Rene & I , said the Same Thing..  Kinda sorta ??

The front tank is the add on tank. So actually the original tank is the one further back. So the fuel would have to go into it and then UP the tube into the larger add on tank. Gravity doesn't really help then.

One thing you don't know is if the tanks have a larger hose connection at or near the top which could allow for a faster fill and also venting of the aux tank. Either the OP needs to "git under there"  and fully inspect what he has or take it to someone who will.

BIG JOE said:

If you fill up with the Gas Station nozzle set at a Very Slow Fill rate.. it might speed up the process ?

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Joe, that shouldn't make any difference. You're only going to get so much fuel through that small line. What it will do is it won't fill the first tank as fast allowing the fuel to transfer over to the aux. tank while the first tank is filling.

fltraveler said:

So the fuel would have to go into it and then UP the tube into the larger add on tank. Gravity doesn't really help then.

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Gravity will help as long as the fuel level in the first tank is higher than the level in the aux tank. Once the levels are the same then it's just a matter of flowing from one to the other.

I know we never filled it near 100 gallons though. The most we filled it was 41 gallons. It had just under a quarter of a tank at that time.

We'll probably have to take it to someone to check it out and see what's really up.

Nobody knows... nobody knows...

There could be a larger hose connection between the two tanks in the middle and another at the top which could accelerate the filling process if the pump handle isn't set to the highest fill level.

But, nobody knows... nobody knows. Because the OP won't call his "friend" and ask how many gallons he usually added when filling and was it a very slow process after adding the first 30 or so gallons.

Op could solve his own problem with a simple phone call. Wonder why he will not do this. After 2 1/2 days of people on here guessing, wouldn't this be prudent?

So you are willing to pay someone to look at it rather than making that phone call? Wonder why?

fltraveler said:

I know we never filled it near 100 gallons though. The most we filled it was 41 gallons. It had just under a quarter of a tank at that time.

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While filling, if you had waited a few minutes once you thought it was full, you probably could have added quite a bit more. Then you'd have to wait again for the level to drop as the fuel is running into the aux tank.  It may take several attempts before both tanks are actually full.

Rene T said:

While filling, if you had waited a few minutes once you thought it was full, you probably could have added quite a bit more. Then you'd have to wait again for the level to drop as the fuel is running into the aux tank.  It may take several attempts before both tanks are actually full.

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True.

I kinda went there, but for now.. until any viable updates, this is still... interesting.

Quillback 424 said:

Nobody knows... nobody knows...

There could be a larger hose connection between the two tanks in the middle and another at the top which could accelerate the filling process if the pump handle isn't set to the highest fill level.

But, nobody knows... nobody knows. Because the OP won't call his "friend" and ask how many gallons he usually added when filling and was it a very slow process after adding the first 30 or so gallons.

Op could solve his own problem with a simple phone call. Wonder why he will not do this. After 2 1/2 days of people on here guessing, wouldn't this be prudent?

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Firstly, I'm a she.    I have already stated all that my friend told me. And all she knows is whatever was told to her when she purchased it. That one flows into the other.  She said it is weird and very slow. Like basically fill, wait, fill again. The reason for everyone giving suggestions here is because people have said things like its very unlikely that there isn't another fill valve or switch or anything. That's why I have been investigating. To figure out if there is something that she was missing that I am also missing. We haven't filled it anywhere near 100 gallons and haven't had any sort of strange drop in fuel that would indicate it being "full" and then all of a sudden "not full" from draining into the other tank. My friend knows nothing more than what vibe already shared with you all. I'm not trying to waste anyone's time or kind help. My husband and I are both super newbies that spontaneously purchased a 23 year old RV. My husband is not mechanically inclined or very knowledgable about automobiles and the like. So we are learning as we go and trying to figure out some of the oddities of this RV.