There is some sort of backflow antisiphon thingamajig on the water line to my black tank spray head. It was located under the sink in the bathroom and when I would hook up to the black tank clean out and turn on the water, water would come out of the thingamajig under the sink.
The dealer replaced the thingamjig and all was well but i check it every time now if I use the clean out.
2004 Monaco La Palma 36DBD, W22, 8.1, 7.1 MPG
2000 LEXUS RX300 FWD 22MPG 4020 LBS