Thanks for the info. No noises but now that I am thinking about it – we were expecting a storm when we left the bus for a few hours and locked and shut everything – but I did leave the ceiling vent in the bath on. That would induce negative pressure. The flapper valve would just suck shut tighter, but I guess the negative pressure could suck gas in through that vacuume breaker?
I will pulse Nick Hessler and see what he knows about black tank venting.
BTW, we virtually never use the washer/dryer – does it have a trap that would dry-out if it weren’t run occasionally?