I have a 2013 Wildwood 26'RKSS with 2 grey water tanks (kitchen and bathroom) and 1 blackwater tank. Both grey water tanks are emptied regularly. Valves are kept closed the rest of the time.
Whenever I shower, the shower drain water bubbles ferociously into the bathroom sink. But when I use the sink, no water goes into the shower. I have just been covering the sink drain with a washcloth to deal with it, but I need help figuring this out. Thanks!