Sewer Smell

all posters giving good advice, I have never smelled boiled over batteries so I don't know about that,but checking the undersink vents is a good idea,also the toilet we have has an overflow tube in it that goes to the black tank. I was told that it is like a P trap also and would evaporate out like the other traps. So see if you have this overflow it could be dry allowing odors in as well. Ours stunk when we first got it too. I flush like crazy when I dump, pour a little chemical in shower kitchen and bath sink drains and treat both black and grey tanks, odor has gone away when driving. I also fill toilet up to go out overflow when making ready for a road trip, just some Ideas.