Picture is sideways
Brass Anti-Siphon vacuum breaker check valve device on the Black Tank Flush System
(also Black or clear plastic)
Water supply comes in via bottom pipe/lifts a disc up sealing off the AIR Cap and then flows out via side pipe down to the spray nozzle in sidewall of black tank
Turn water OFF....disc drops allowing AIR to come into the tank side of device and closes off the supply side of device
Prevents cross contamination of waste water into potable water system and breaks any vacuum so no siphoning action can occur
That is what it is designed for and required by 'standards' for RV MFGs
What they do in reality is provide a means of water flooding into RV (disc not closing ...water flows out top cap) and Black Tank STINK coming back into RV (check valve failure)
Waste tanks..........fill them/dump them/add few gallons back in to keep tank-stuff wet----repeat
They are holding tanks and get dirty very next time you /pee/poop
Waste of good water flushing a waste tank.