UPDATE: No more smell. After thinking about out it and reading the comments, I decided the cool air was coming from around the pipe. It was a good volume of air and I think it was pressurizing the vent cap and sending it down into the tank pressurizing the tank.
I took some strip caulk and sealed around the pipe until I could not feel any cool air. We just got back from a two week trip and no smell. I couldn't smell chemicals or sewage standing there holding the toilet valve open looking at sewage.
I knew something was wrong because we've had two other campers and never had this problem.
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Sorry, don't know how to post pics
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