Probably is the overflow drain for the fresh water tank. There may be a 2nd on the other side of the coach.
You probably have the 4way Anderson Valve in the UDC and you switch it to TANK to fill the fresh water tank. Because the water is under pressure from the faucet (as opposed to the older style "gravity fill"), you need overflow valves to keep the tank from being overfilled and damaged.
You may find additional useful info about your water system in our owner-written Water Systems Guide.
If it is the vent for the fresh water tank, most likely is, the fitting needs to be pointing down. That way it won't catch dirt and anything else that could find its way in your freshwater.
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