[JRTJH]

The only times I've had "sewer smell" in the trailer is when the bathroom exhaust fan is on and we flush the toilet. When the flush valve on the toilet is open, the fan "pulls" air down the vent stack, through the tank and up the open toilet valve.

Above is the primary cause. The other, at least for us, is when I reverse the exhaust fan and use it to pull air from above the RV, down the vent and into the bathroom. The black tank vent cap is "right behind the vent fan" so when I reverse it, anything that's "floating up the sewer stack and out" is subject to be pulled back into the coach. This is especially problematic "IF" there is sewer gas flowing from the campground sewer system, up the sewer hose, into the black tank and out the tank vent stack. (This is the reason to not leave your dump valve open on any tank, but always closed on the black tank).

Other than these two conditions, we've never (at least that I can remember) had sewer smell in the bathroom (other than when the toilet is "in use".....