Uh-Oh

I have a 2016 Dutchman Denali 335rlk fifth wheel.    It has a panel where I can hook up a hose to flush the black water tank.  I was filling the tank and forgot about it.  So, guess what, it over filled.  My wife heard water so I ran out and opened the gate valve on the black water tank.   
No water backed up into the toilet.    But, it appears that there must be an overflow valve?    I'm hoping that is the case.    Does anyone know about this feature?   

There is a black plasticized cardboard liner riveted to the frame under the rig, so I cannot see the black water tank. Where the 4 inch ABS plastic comes through that liner, there is expandable foam sprayed around that opening.  That foam got wet.  I pushed up on the liner and could tell it was holding water. So, using a retractable blade, I made a small hole in the liner to let it drain.  I made sure that I didn't go in too deep as to puncture anything other than the liner.

I doubt there is an overflow valve/hose. What probably happened is the seal on joint where the pipe from the toilet to the blank tank failed. There's a good chance it wasn't sealed well from the factory. Personally, I would start taking out the screws that hold the Corplast (black plasticized cardboard liner) and get the underneath aired out well. It probably wouldn't hurt to do some cleaning up under there. Don't just let it try to dry without opening it up. You can also carefully cut a section out with a razor knife. Just cut three sides, creating a flap. Keep the uncut side to the front of the trailer. Tape it back into place with Gorilla tape.

Most black tank flush setups have an anti-siphon valve in the line somewhere. Seems like that's where the water came from as your holding tank filled and water came back up the tank flush line.
The anti-siphon valve on my trailer is under the bathroom sink in plain view. Yours may be in a similar location or at least easy to see so it can be replaced if needed.
It sucks that the tank cover is riveted in place but they can be drilled out to free the cover. Along with cutting the expandable foam.
You definitely want to get the cover off so everything can dry out and not get moldy, etc.

Ther are only 3 ways into/out of the black tank on most RV's 4 on mine

THe toilet (1 and 2) the dump gate and the roof vent.

NOW.. this is the part you WON't like.

Though in the Movie RV we see a "Fecal Volcano" with 'Stuff" Shooting out the roof.. don't happen that way

THe pipe on the roof is much larger than the pipe up to the roof. the actual vent pipe "Rattles" around inside the visible pipe as it were.. At least on my rig. If you remove the "Hat" from the "Stink Pipe" (Terms are descriptive) you will see a smaller pipe insiide. IT may have run down inside the wall where you can't see it.. Just so you know.    WHy I NEVER leave water running into that connection unattended. I sit there and wait till I think it's ready to re-dump.

John From Detroit said:

Ther are only 3 ways into/out of the black tank on most RV's 4 on mine

THe toilet (1 and 2) the dump gate and the roof vent.

NOW.. this is the part you WON't like.

Though in the Movie RV we see a "Fecal Volcano" with 'Stuff" Shooting out the roof.. don't happen that way

THe pipe on the roof is much larger than the pipe up to the roof. the actual vent pipe "Rattles" around inside the visible pipe as it were.. At least on my rig. If you remove the "Hat" from the "Stink Pipe" (Terms are descriptive) you will see a smaller pipe insiide. IT may have run down inside the wall where you can't see it.. Just so you know.    WHy I NEVER leave water running into that connection unattended. I sit there and wait till I think it's ready to re-dump.

Click to expand...

4. The black tank flush. Water can get in so water can get out.

Ok,  there are several points made that I will try to explain what I have found.

The  issue of an antiphon valve.  If the water were to back up that far, it would seem to me that I would have some water inside the trailer but I am not seeing any trace of water anywhere.

Also,  I did not mention anything about water coming up the flush line.  I connected a wye on the city water connection and then a very short line that stays connected to the flush line input connection.

The person that mentioned the seal failing between the toilet and the line makes me wonder if that were true, then continued use of the toilet would lead to continued water leaking, which I will definitely watch for.

I really do not want to remove the entire Corplast sheet since it runs the length of the 38 foot 5th wheels under carriage. 

Seems like I got a myself a huge job ahead  of me. (laying on my very old back).

Hammster said:

4. The black tank flush. Water can get in so water can get out.

Click to expand...

No that is inlet only. check valves or other backflow preventers. If you have pressure on the inlet fitting (he did the "Exit" alve is closed or .. Well 50 PSI on water will lift about 100 Feet

Nick Pasich said:

The person that mentioned the seal failing between the toilet and the line makes me wonder if that were true, then continued use of the toilet would lead to continued water leaking, which I will definitely watch for.

Click to expand...

The drop pipe from the toilet often extends into the tank a couple of inches through a simple rubber seal/grommet. As long as there's space in the tank, flushing the toilet will not leak, even if the seal is completely missing. Overfilling the tank though, will leak since the seal setup is not designed for pressurized use.

John From Detroit said:

No that is inlet only. check valves or other backflow preventers.

Click to expand...

So, these things never fail? Or their connections never leak? Pretty sure they do.

Hammster UNDER the specific conditions given in the original post NO they never fail

Now once you shut off the park water and remove the positive pressure.. Yes it is possible but the type of check valves used are not prone to failure  Or to be more specific not to THAT type of failure. They do fail but usually on the fresh water side.

My Montana has, in BIG RED letters.....dump valve MUST be open when using this (flush) fitting.  Probably good advice!  Flushing with the dump closed I doubt would do anything useful, since the deep water may reduce the spray of the flush fitting inside the tank.

meternerd said:

My Montana has, in BIG RED letters.....dump valve MUST be open when using this (flush) fitting.  Probably good advice!  Flushing with the dump closed I doubt would do anything useful, since the deep water may reduce the spray of the flush fitting inside the tank.

Click to expand...

Here is how I use my tank flush.
I drain the black tank until I don't hear any more flow down the stinky slinky. I then close the black tank dump valve.
With the tank flush connection connected to a hose and to the water supply I start flowing water into the flush. I monitor the black tank level until the full light starts to flicker. I turn off water to the flush and open the black tank dump valve. You can really hear the whoosh of water coming out of the black tank. I do that twice. After the second time, my black tank always indicates empty.
So, yeah, the tank flush with dump valve closed may not be recommended, but it works really well to clean and flush the tank. You just have to pay close attention to the tank level while doing it this way.