About 10 years ago when we bought and were extensively remodeling the stick house where we retired to, DW wanted grab bars just in case. I had the builder put 2 X 10 backers in the wall where bars would go in the shower and near the toilet. After the ceramic tile was done, I installed grab bars anchored to the 2 X 10 backers with 3 large screws at each end. I think this is probably the ideal solution for grab bars, given how difficult it would be to line up both ends with studs. All these years later, the grab bars are pretty handy.
After my mother had a stroke, I installed 3 of the suction cup type fastened to the ceramic tile in her bathroom. In the 2+ years before she passed away, I had to refasten one end of one grab bar one time. These things hold incredibly well to a smooth surface.
Based on this prior experience I decided the suction cup type is fully acceptable for the motor home. I’m confident it would pull the shower surround off the wall before the suction cups would let go.
As an added benefit, it won’t mar the surround if the next owner doesn’t want it there. If I don’t like where I put it first time, I can reposition it.
This is the one I’m using:
http://www.lowes.com/pd_191936-94039-S10255_0__Wayne