[sourdough]

It's the backflow preventer for the black tank flush. It may be installed backwards or defective. You can get them at a hardware store, or, I've seen RVs where the preventer is removed and just replaced with a piece of tubing - as long as you keep the water lines up high with the loop. If it were me I would just buy another valve. It could also be that something is loose or installed improperly. You will have to look at it more closely to determine that.