On ours, there is a cutout on the paneling that you have to kneel down low to see. I just looked at a diagram of your coach and the closest water line would be the guest bath...unless they put the tap/valve on one of the outside compartments. Have you looked at the compartment with all the water hookups. The valve, if I recall correctly, is white and has the regular size refrigerator hose.
I didn't winterize the fridge since ours will be hooked up to shore power throughout the winter. We have an oiled filled electric heater set to about 40 degrees. We had an early winter and I tried the coach's furnace and went through an entire tank in no time.
2014 Georgetown 351DS
-TruCenter -Front/Rear CHF -Hellwig Links -Tiger Trak -Ran McNally GPS -ScanGauge -Truck Systems TPMS -5 Star Tune