Black water Tank back pressure
In my 2018 Thor Hurricane 34J, I experienced a lot of back pressure in the black water tank which resulted in the contents of the toilet being blown back at the person pushing the toilet flush pedal. As you might imagine, this was a terrible situation especially when the grandchildren hit the flush pedal as they are so much closer to the issue... After many conversations with Thor tech support, Camping World service department and reading many forums, the problem was identified.
The bathroom sink drain is plumbed directly into the black water vent pipe which runs down and then horizontally toward the passenger side of the coach and then down to the rear side of the black water tank. This was done to help flush solids down to the deep end of the tank. The coach factory level is set slightly off to the drivers side so that rain water on top of the super slide will run away from the coach. The result is water from the sink creates a trap in the horizontal section of pipe before it flows into the tank. Thus, prohibiting the black water tank from venting and therefore causing pressure to build in the black water tank.
Solution! I took the motorhome to Camping World and had them re-plumb the sink drain into the grey water vent which runs right beside the black water vent. The grey water pipe runs straight down into the grey tank, so no trap issues. This will not be covered by warranty, but well worth the money. $13.00 in parts and 1 hour of labor. The sink drain did not provide enough flow to flush solids, so moving the drain had no effect on the system as far as moving solids. Just be sure to use your black tank flushing system each time you dump your black tank. I hope this helps somebody out.