Well, if that is a port to connect a siphon tube to siphon antifreeze out of a gallon jug, then there has to be a connection to the intake side of the water pump, with some sort of arrangement (usually a two-way valve) to prevent the pump from simultaneously sucking up both antifreeze and water/air from the fresh water tank. The question I have is where is the valve? It should be near the water pump.
Happy Camping! ///// Richard D.
2006 4x4 Ford 250 SD / 2007 Flagstaff 827 FLS
One very patient wife and one furry child who travels with us. Forty-two years of trailering and camping, and I still have a blast.