When we bought our 2007 3124 KS new it came with a shower holder screwed to the back wall...the one in front of the rear bed. This item is readily available from camping suppliers for example Camping World:
http://www.campingworld.com/shopping...ll-mount/34913 They used to come in a cream color for about 2.79 and that may still be available at other rv supply stores. The one that came with my TM is cream colored.
I have never had any damage to this bathroom wall due to moisture. I did line my walls with thin clear vinyl like Commodore 47 did. Since the water sprays forward there has been no problem.