I have not looked under the sinks at out Palazzo. In our fifth wheel, they used an air admittance valve or STUDOR valve. They installed the cheep one that only relies on a rubber flap to make the seal. When I found the problem, I replace it with the STUDOR valve that meets most stick and brick codes. It has a spring. Could purchase five of the cheep ones for the price of the good one.
I always add Happy Camper to the gray tanks and well as the black. It eliminates odor. If you ever had to change a gray or black tank valve, you would too! The gray smells much worse than the black, and don't ask me why I know, but man it does. After I used Happy Camper, the wast valve replacement was never an odor problem. I do not know if other waste treatments will do as good a job as Happy Camper, but It works, so I use it even in my Palazzo.
I hope I did not violate a form rule by mentioning the product I use. I have no affiliation with them.
Thor Forums
