Was looking under the sink contemplating adding another outlet on the counter. I saw this random pipe and port inlet. What’s it for?
Drain vent, so when you empty the sink it doesn’t siphon the p trap, and ending up smelling the gray tank.
That is a air inlet valve for your sink drain, it is a one way valve that lets air into the drain pipe
It’s the pipe that over flows with water when you son decides he should clean his shoes and fills up the gray tank.
Air admittance valve as others have said. Also the black piece behind and above that is your black tank flush backflow preventer
Pushes bad smell out instead of in.
Air Admittance Valve (AAV).
They often fail and cause drain issues aka vacuum locks.
Source: What’s this pipe and thing
