Sometimes it helps to see what the RV builder used to mount stuff to the wall.
For towel bars, I made up some small spreader plates of thin (1/4" or so) plywood, looking vaguely like blank switchplates for lights switches, glued them to the wall (mainly to keep them in place while working), and then used plastic wall anchors through both the plate and the luan of the wall. My theory was that they would help spread the force of the mount out a bit and so prevent puncturing the wall, and the added thickness would make the wall anchors more likely to hold. Thus far it's worked out okay.
For many things, command strips would work nicely and be removable if desired.