Your black water tank only has a few exits for gases.
First is through the toilet itself. If your toilet valve is holding water in the bowl (and there is always some residual water after you flush), then the valve is water-tight and thus gas-tight and no vapors are coming up through the toilet.
Second is through the P traps of your bathroom sink. I have heard that in some RVs some sinks dump into the black tank, even though it is technically "grey" waste water. Those P traps are designed like they are to hold water in them, which block the flow of gases through the trap. If you haven't use your sink in a while, the water in the P trap can evaporate out, leaving an open passage from your dump tank out through the sink drain hole. I have heard it said that this water in the P traps can also slosh out of the trap from driving the RV/trailer. The fix is run a little water through your sinks when you get where you are going.
Third is your vent stack. Your tanks are vented through PVC pipe going up through the roof. If the vent pipe becomes disconnected or broken inside the RV, then your tank will be venting into the RV. Likewise if you have a vent or window open near where the vent exits the roof, then it is possible to suck vent gasses back into the RV.
Steve
