We had an '11 355QBQ last year for the winter. This was our first time winterizing tour '12 365SAQ. I had no idea what this valve was either. Took me a while to figure it out too. Finally got up enough nerve to turn it and see what happened. It worked! Guess I should've came back to forum a little more often
Anyway, worked like a charm an used almost 6 gallons to winterize mine as mine has the outside kitchen and w/d prep. Once I figured it out, didn't take long at all!
2012 Sandpiper 365SAQ weighing @ 15k, Onan, 2nd air, slide toppers, TST
2013 F350 6.7L Lariat 4x4 CC DRW,Viair 1007, Ride-Rites, Edge CTS, B&W
2001 Chevy 3500CC DRW 4x4, 8.1L, 4.10 gears w/ Detroit TrueTrac
2001 Ford Excursion Limited 4x4 7.3L,V/B Spring Mod, ProComps, Hellwig Swaybar, & other 'Necessities'