Is there a plumbing diagram or section devoted to plumbing in the owner's manual? On our particular Jayco, there's a manual page, which is confusing, a patchwork of valves under two cabinets (also confusing), and Sharpie notes on the floor. Put there by previous owners. Apparently they were...confused...
It might help to be sure that the Shower and Outside Shower are OFF at the Knobs, not just at the Shower Head.
With where we're located, and because we use our rig year round, I'm no expert on Winterizing. But I wonder if a valve involving Winterizing isn't involved in this problem.