I would give up on the fiberglass mounting, compared to metal, it will never work to as well as a metal.
Here is what I have used on my trucks. yes you can use this without damaging paint. There are 2 different ones (both found on same link), depending on what your antenna needs to mount to.
I would put this on front hood, forward enough that antenna does not hit overhang of camper. I have used them for years for Amateur radio, but they also work for CB radio