I just winterized (again) our 2012 Discovery 36J, and the access is through a panel in the closet - remove 6 screws, remove the panel, and the water valves are visible, though a little hard to reach if you have "fat fingers".
Nice to see other Discovery 36J owners on the forum!
Happy Thanksgiving to all...
Joe and Debbie, Emma the Aussie Cattle Dog who adopted us
2012 Discovery 36J, Blue Ox and Air Force One, 2010 GMC Acadia Toad
USAF (Ret) WDØM http://wd0m.com