I've heard these work really well. I haven't tried them myself, since I don't know what is going to happen with my RV.
http://www.amazon.com/dp/B001FC7DUC/...I1KLRKVBWWZIOR
Another thing, I'm not sure about yours, but on my RV the lavatory (bathroom sink) drains to the black tank, so if that P trap runs out of water, you will get black tank fumes.