It's to suck in antifreeze when winterizing.. Your valve is currently closed, when you winterize, open the valve and close your other inlet valves, turn on your water pump then run the antifreeze through all your taps, showers, toilets..
TT: 2015 Jayco Jayflight 26BH Elite
TV: 2014 Ram 1500 Sport, 5.7 HEMI 3.21, Firestone 1000# air suspension. WD hitch w/800# bars
SOLD: 2012 Jayco J-series 1007