Help with new engine decision

If it is old dirty camper smell, it probably is mildew. If it is dead animal smell, then it is propane. On my coach, anytime there is a dramatic temperature change, I get propane smell. It is caused by the valves seeping during thermal expansion. Since it only happens in the winter, I don't worry too much about it. The other thing I do is since I have a MaxxAir vent cover on my bathroom, I leave it open all winter long. That prevents any propane smell from building up. It does make the bathroom dusty, but the cover does not leak.