Since it only happens with city water and sink (I'm thinking it only happens with sink) I suspect a drain or gray tank issue,,, If you have 3 tanks that one is the Gray. If you have a "T" handle like the black tank dump handle if you can see where the rod goes. If it goes to a gate valve in the 2-3 inch class (Like the black tank dump) that's your 3rd waste water tank (or perhaps the 2nd if it is a trailer, some of them have a 2nd valve in teh dump line
Black Valve, Gray Valve, Wash Valve all connected via a manifold to a Main Dump valve which must be opened first.
