Look under your sink for something that looks like this
It's what most (all that I've seen) RVs use to provide a proper vacuum brake for the P-trap. They get old and fail letting odor into the area under the sink.
Link to the Home Depot versionThey're cheap and rightfully so, we had one that was bad right out of the box. After replacing all the ones in our coach we now have no smells at all, even traveling with the roof vents open and the negative pressure in the coach doing so causes.