Another confusing electrical problem

I hope this isn't long winded but want to thoroughly describe problem. I am hoping it is something simple I am missing. We had furnace firing problems which were being attended to. The last remedy performed seemed to be working, but was instructed to "test" with different parameters. We had the 2017 Class C home, so we did not have 50 amp shore power. We utilized the generator to supply power. We ran the generator, tested furnace (and ran as well as slideouts, air, etc.) and everything ran great. then shut down. We were to leave on another trip, so as I usually do, I unplugged the refrigerator form the coach plug and plugged into extension cord to house (not the outlet but the appliance). I mention this in case it is relevant to the problem. Anyhow, the next morning while preparing to leave, I figured I would test the furnace again. Fired up the generator, but  the blower didn't fire up (therefore furnace controls were not getting power) so no furnace. First thought thermostat, so turned down and tried front air conditioner ( same thermo as furnace) which also had no power, so must be something with thermostat right. So since  the back  air conditioner is on a separate thermostat tried that - no power. So couldn't be thermostat. Everything else works on generator. Checked circuit breakers, fuse box etc. nothing seems amiss. Am I missing something like an in line relay or fuse or something that just needs to be reset? The reason I mentioned the fridge plugin previously was that when I fired up or shut down the generator (can't remember which) my wife thought the lights flickered in the house. But like I mentioned the appliance was plugged into the house  extension cord, not the coach. Is there some " back-channel" on the fridge that would mesh both the house power and generator power and mess something up? I am not an electrical whiz but that just doesn't seem possible. but with motorhomes, anything seems possible. Thanks for any advice or pointing in the right direction.

My first thought is that it might be related to your DC 12 volt system.  All of the things you mentioned as not working also require 12 volts for the thermostat.

Did you maybe turn off the battery disconnect or could the battery be low.

Just my first thought.

Edit:  Now before you start thinking that I know more than I really do, I must tell this short tale.

A couple of weeks ago Mario, ( a friend that does some work for me), and I were working on the floor in the coach.

His 3 year old son was with us.  A three year old has to push every button and turn every switch of course.  All of a sudden both Air Conditioners stopped.  We looked up front and the little guy had his hand on the battery disconnect switch by the door.

The A/Cs will not run when the batteries are disconnected.

That's my story and I am sticking with it.

Another edit:  The rest of the story....

The little guy also turned on the Scare lights, (four outside security lights).  I did not notice because it was daylight.

A couple of days later I went to start the diesel engine.  No power, I pushed the jumper switch and it started off the coach batteries. 

I went down the hill to the battery store and bought a huge starting battery for about $300 and paid one of the guys to stop by and install it at my house on his way home.  I also installed a Battery Tender, (the good one not the Harbor freight one).

After installing the new battery I saw the security lights were on.  Probably did not need a battery but it was probably original so I guess it could be considered preventive maintenance.

Picture below of the old one.

How many volts is your battery reading during this?  Not how many lights are lit on the RV monitor. The actual battery volts

Just curious why you're plugging your refer into an extension cord and not just plugging in the RV. That would keep your batteries charged up also.

The only thing that 120 volt and 12 volt have in common on the fridge is the control board (And there it's just an "Is 120 volts present" sensor) and a control relay 12 volt controls it, it switches 120 volts.

No way can that feed back.

The furnace however is 100% 12 volt. Power is sent from a fuse somewhere to teh control board. from there it goes to the Thermostat and .. if cold. back. a Relay on the control board closes and fires up the blower.

NOW for some reason.. If my Control board is in position.. I get a short that's shutting it down. By watching an indicator you don't have I see the voltage drop... I'm still working on it but if I "Rattle" the board.. all is good.. Since I usually heat electrically. not a problem.

I also suspect a 12V system issue.

1.  Verify battery disconnect is closed (battery connected to the coach)

2. Measure voltage at the batteries with a meter -NO SHORE POWER OR GENERATOR

3. measure voltage at the FUSE box, where the battery feeds the fuses - NO SHORE POWER OR GENERATOR

4. Start the genny and repeat measurements 2 and 3

This will check battery status, converter / charger status and disconnect status.

Hopefully, this will help in the search.

hello all-
Thanks for the replies so far - I did try some of the suggestions previously, but will attempt again (am away right now).Batteries are all fine and voltage is perfect (based on monitor) - but several thoughts based on your replies -

I tried with (or at least thought i did) battery switch on and batterry switch off same result.

Was told air runs on generator (120) so battery on isn't required, but I see is required for Thermos and furnace (correct?)

Was also told 20amp plugin to 50 amp coach was not a good idea, that is purpose of extension cord to fridge.


Again much appreciated  - will try everything again when I am at coach.

Do not trust the monitor.  Check the battery with a separate meter.  Without being plugged in to any power source, the battery should read around 12.6V.  When plugged in, the converter (charger) should push the reading to 13.6V.

There is no problem plugging into a 20A circuit.  Just remember that the extension cord should be as short as reasonably possible, and the length and wire gauge may further restrict the power available to the camper.  Clearly, the camper can not draw 30A of power through this setup without popping a breaker. Also, while you MAY be able to draw 18 - 20A without tripping a breaker, if the wire is too small or too long, there will be a voltage drop that can harm the fridge or heat may damage the cords.  Just be aware of what the draw is for what you want to run.

Thanks everyone - here is the deal. I knew the solution had to be somewhat simple due to the fact that everything but the furnace and ac  (thermostatically controlled) worked . All circuit breakers were good and voltage from batteries was good. (
But with the knowledge gained that 12v probably controlled I went back to basics. Looked at fuse box, nothing labelled for thermos or furnace. So I pulled both thermos covers saw fuses, said ah ha must be on individual 12v circuit - but both fuses good. Frustrated went back to fuse box and decided to check each and every fuse. Luckily part way thru procedure discovered blown fuse - it was labeled appliances!! changed and everything is good - Stop laughing.
Thanks again

Thanks for the update!

One of my first tasks with my FW was to pull each fuse individually and see what did not work.  It was somewhat accurately labeled, but now I KNOW which fuse controls what!

I had another incident where a fuse blew, but stayed sufficiently connected to NOT light the blown fuse LED.

I'm not laughing.  I'm glad you found it and it was a simple, cheap fix!

Appliances. Well by definition the Furnace and A/C (And Fridge and Water heater) are all appliances.

Cryptic. that is how they label 'em. Cryptic.