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...
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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
https://wd0m.com