Winterize connection

More likely it's an alternate suction port for the built-in pump to let you pump antifreeze throughout the system. There are probably one or two valves somewhere to switch the pump between this port and the usual intake from the fresh water tank.

If you have a fill valve to fill the tank from the usual city water connection (as opposed to only having a separate gravity fill port), you should also be able to use this winterize port to fill the tank from an external container using the built-in pump: hook a short hose up to it and put it in the water container, set the valves to winterize (so that it pumps from the port), open the city water fill valve, and turn the pump on.

I have the same type winterize connection on my 5th wheel. On mine, I connect a short hose to the winterize connection with the other end in a jug of RV antifreeze. There is a switch nearby for "winterize" and "normal". I move the switch to the "winterize" position, turn on the pump, and it sucks antifreeze from the jug and thru the water lines. Takes nearly 4 gallons of antifreeze to fill all the cold and hot water lines, including toilet, washer/dryer, black tank flush, and outside shower lines.

Thanks for the replies, I'm sure you are both right and it makes sense now.

I have an 08 monaco with the same connection. It's an alternate suction for the pump. Throw the vale, hook up a hose and put the free end in some antifreeze, turn on the pump, run faucets till pink. Will take about 4 gallons of pink stuff.

Be sure to drain and bypass the water heater first.

It is in the manual, at least is in mine, under winterizing in the plumbing section.