black tank not venting?

SJ-Chris wrote:
It just occurred to me that you could have BOTH problems....the tank vent could be 1/3rd into the tank AND you could have a small leak in the connections to the black tank (either the vent, or the toilet, or maybe your bathroom sink). If it is a small leak, then perhaps when the tank is less than 2/3rds full MOST of the smelly air vents out the vent pipe to the roof. But when it is more than 2/3rds full, maybe the vent pipe is submerged and now the stinky air cannot go up the vent pipe and finds its way through a leaky black tank connection (and then the smell drifts up into your RV).
Good luck!
Chris

Chris - no leaks at all... not ever and it never smells EXCEPT when we flush and the tanks reaches about 2/3 full. When it is 1/3 full, even if it has been sitting a while, there's no real noticeable smell. That is why I think the most likely scenario is that the factory glued the vent pipe in place poking too far into the tank as suggested by others as well. At that point, as you say there is no longer any vent tube for the stinky air to escape so it comes right out of the toilet when flushing... and much worse if the vent fan is on in the bathroom. That makes total sense to me.

As for the air pressure that built up in there, it is basically a closed space the the water level is above the bottom of the vent pipe and with the toilet valve closed and water on top of it, the pressure would then build in the open space with no way to escape and would blow out the toilet when it opens. "Geyser" was much too strong a word by wife gave to it. I think it was basically like air pressure that built up and then blew the flushing water up in a spray into the bathroom and onto my wife's shirt. The tank had been saniflushed twice with 20+ gallons in there, so the water in there wasn't particularly dirty but anyhow, I doubt there was much of the standing water that sprayed out when the air pressure escaped... just the new fresh water trickling down the bowl when she stepped on the flusher. That's how I see it. I am not much inclined to tackle repairs myself. I know some mobile techs who will come over when the black tank has been emptied and saniflushed and try to raise the pipe up so it barely enters the tank and then reseal it.... and then presumably shorten the piece of pipe coming up thru the roof.

I'll post back to the forum after a tech has tackled it. Meanwhile for now we just no we have to dump and saniflush each time the tanks reaches about 2/3.