Trying to locate vent for black tank in our floor plan. I have located the vanity sink location. Another in the kitchen area. The Venturi is next to the shower dome on the opposite side of the coach. Wasn't going to get back into plumbing until next month but the priority has changed.
Black tank does not have a screw-on under-sink vent like kitchen and bath gray drains.
Black tank relies on just the PVC stack vent pipe from tank up through interior, then sticks out the roof.
No idea why your toilet would spit back other than being too full. If the PVC stack is clogged taking a water hose up to roof and running water down into stack seems like it would clear things out. If you have a top cap, maybe it is blocked.
I guess if the black tank gets too full, it could block vent air flow, as PVC pipe may stick into tank too much. If it only happens when near full, just empty black more often.
If your toilet is not over black tank, run a hose into toilet to clear the angled pipe.
With every weekly dump, we use a water hose through window to clean toilet, flush shower & sink P-traps and flush tanks with lots of water flow. We have a dedicated 50' 1/2" water hose with inside end cut off to eliminate scratches and preventing hose from catching inside drains. We also use a wireless intercom for inside person to communicate with outside water control person.
Thanks Barry. I think I was thrown off by the Venturi being on passenger side but then the tank is transverse and so it makes sense. Hold over from bumper pull. I will clear the line tomorrow. Like the way you clean all plumbing and will do that going forward. I had emptied the tank a couple of days ago with a macerator to the house clean out. When the macerator spun up I thought it was empty.