[jsb5717]

Look under the sink for a vent in the drain plumbing. It's designed to pull air in to allow for better flowing without allowing odors into the coach. They tend to go bad and need replacing. They are snug but they do unscrew and are easy to replace. If there is no vent then I'm back to the drawing board.

I bought these to fix the odor in our kitchen area. Problem solved

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

Also, I suppose that you could eliminate the ptrap in the basement and install ptraps under both sinks if the vent thing doesn't help.