OEM towel bars & etc use screws, so it shouldn't a problem as long as you're mindful of the screw length AND avoid areas where wiring, plumbing, & etc might be located.
Another inexpensive option is to use plastic over-door hooks inside the shower... have used this method in some of our smaller RV's (including out current Laredo)
to save space & keep hanging items out of the walk-way. Here's one example:
http://www.lowes.com/pd_339369-1783-...oks&facetInfo=
As byrdr1 does, we also use double-sided tape or 3-M adhesive strips (& photo hangers) for light-weight items. For heavier things, we like (industrial strength)
adhesive velcro. Never had any issues with anything falling... ummm, "knock on wood".
Terri, the Chevy co-pilot