I think you're talking about some sort of a sealant. There are a few reasonable options to use. Butyl rubber tape (as is used for RV window frames, trim strips, etc.) is one option. Particularly for small odds and ends like baggage door hold-open clips, plumber's putty works pretty well and is easy to get and inexpensive. Some sort of a urethane caulking compound (Geocel Pro-Flex, OSI Quad, etc.) is another reasonable choice; though avoid silicone as it is very difficult to renew or touch up if needed since basically nothing will stick to cured silicone. Dicor non-sag compound would also work (but not their self-leveling lap joint compound, which on a vertical surface will self-level its way down the wall).
In any case, the main goal is not so much to keep the thing from touching the fiberglass as it is to ensure a watertight seal around the penetrations through the fiberglass.
If you have something reasonably solid to mount to (a stud preferably) screws are perfectly fine. Even if you're just mounting to the exterior skin, they'd probably be okay; it's not like you'll be driving with the TV attached, one hopes. Factory installations probably have a backing board or something similar built into the wall for the screws to be driven into, but adding one after the fact is not practical. Bolting through the wall means having a chunk of metal with no thermal break going through the wall, which is hardly what one would prefer if it's going into the heated/cooled interior of the RV, besides looking rather hokey on the inside.
