I screwed two levels to the front and sides of my trailer using stainless screws. Drill a pilot hole to fit the body of the screw, put a small bit of caulk in and around the hole, press the level into place, and carefully screw in the screws.
Most levels have a self stick backing on them which I use but use the SS screws also. I have never had a problem with them coming loose, shifting, or the fiberglass cracking. Just get the pilot hole the correct size and do not over tighten the screws. :) Also, make SURE the trailer is level before installing the level indicators. I use a carpenters level on the floor but many use a smaller one inside the fridge to make sure the camper/fridge is level.
Barney