I think its your winterizing port you can test it by taking a clear jug of water open the valve put the hose inside and start your potable water pump with a faucet open if the water level goes down you have your answer.
Jake my sidekick (yellow Lab) 10/04 - 05/20
2017 RAM 2500 CC 4X4 Cummins Diesel
2016 Flagstaff 26 FKWS
AF&AM & El Korah Shrine of Idaho