I dont mean to hijack but we have a similar problem. We only get the bad smell after returning home from a camping trip. I have checked the black tank vent, replaced 3 vacuum breakers(inline vent) and tried putting rubber stoppers in all the drains. We have a double sink with a vacuum breaker, shower has one and the bathroom sink has one. Did I miss one some where? We dont have any other drains. The toilet flap holds water. Im going to replace the seal between the toilet and floor, is there any other seals in the toilet that may need to be replaced?