My father in law recently purchased the same type of rig and was having similar issues. We found a well designed dump station (one with a large sloping basin to the drain) and went in the drain pipe with a drain snake. Ended up pulling out about 50 of those thick brown paper towels that do not decompose. Prior to our cleaning his black was not even draining. He still took it into the dealer and they pulled even more of the towels out. Just told me he is still having issues and will be taking it back to the dealer again. I told him I suspect the towels may have damaged the valve seal.
A very weird ordeal overall.
