Have you considered an over-the-wall unit such as this one?
http://www.trailmanorowners.com/foru...&pictureid=449
It simply hangs over the top of the folding wall above the tub. Holds the shower head, soap, shampoo, and whatever else you need. No mods to the camper at all, no screws into the walls, etc.
The picture shows the one I built, but there were several similar designs put forward, way back in the early days of the forum (2002, maybe?) You can start by looking for posts by a member named Hal. Plastic parts come from WalMart, add a few stainless steel nuts and machine screws from the hardware store, and you are done.
Bill