I will refrain from my usual "there is so much prep work involved why use a cheap paint in the end" rant here
but I do want to mention that you should solvent wash the fiberglass (assuming you are painting over original gelcoat) to remove the mold release wax (which does lurk even after so many years).
You want to do this process before sanding, as otherwise the sanding just grinds the wax further in to the camper. The procedure is to wipe a section at a time, and constantly fold your rags over to expose clean sides, so that you are not just spreading the wax around (or use the disposable "cloths" that painters use such as Kimwipes, etc. -- check whatever you use to be sure it is appropriately "clean" of contaminants as a cloth).
The trailer won't seem "waxy" but the mold-release wax is does not "show" like that.
Interlux Solvent Wash #202 is an example of such a solvent, although I'm not suggesting that you need to use that particular one. However you could Google it at their site of www.yachtpaint.com
Happy prepping (and then that last little step at the end... painting)