For installing things on the interior you can first remove areas of the carpet and clean the surface of glue residue. Then install blocking against the fiberglass surface using thickened epoxy. The interior surface of the fiberglass on a Trailswest
Campster is rough which means that VHB tape is not going to work for attaching anything onto the interior surface. Just because a person with a different trailer says it worked on theirs does not mean it will work on your trailer. VHB tape will also not properly adhere to raw wood that has not been coated with a sealer.
For installing heavy items such as one of the awnings that has a metal housing you can install blocking on the interior for the mounting screws or bolts to grip into. Or for thru bolts you can drill thru and on the interior use thick fender washers or custom made metal backing plates. The fiberglass shell on its own is not substantial enough to bear the
weight of a heavy awning that only has a very few attach points so for that type of installation you need to distribute the load across a wider surface area on the interior surface by one means or another.
If you have an unpainted exterior you can install some types of the long awning rails with VHB tape. There are many versions of VHB tape. The 3M company has a website with recommendations of the various types of VHB for use in various situations on various kinds of surfaces. That site also discusses the required surface preparation and what primers might be needed to insure a proper bond.
There is more than one type of awning rail around. Some hook into clips. Some are long with a flat back and screw holes every so many inches. Some are flat with no screw holes. Some have raised edges, some are curved, others are L shaped. There are also vinyl awning rails. Some awning rails are ridged, some can be conformed to curves. This is because there are many types of RVs and other vehicles including boats that use awnings. The good thing is you have choices, the bad thing is you have to do research and make a decision.
Just remember that a
Trails West Campster is not all that tall. In the long run it might be a lot easier and less expensive to use a popup shelter for your shade structure.