What you do with the exterior depends on how badly oxidized (chalky) the original gelcoat finish is. If it's still in fairly good shape, just wax it. Any regular automotive wax will do to protect the finish, but for a better (shinier) result use a marine fiberglass wax. Meguiar's and 3M both make suitable products.
If it's pretty chalky, both Meguiar's and 3M also make fiberglass restoration kits that include oxidation remover, polishing compound, and wax. Here's a link to the Meguiar's version: Meguiar's Oxidation Removal Kit
Another option for a high-gloss appearance is Zep floor wax. You'll want to clean it thoroughly and remove oxidation first. It wipes on, no buffing, and 4-6 light
coats make a deep shine. It does have to be stripped and reapplied periodically. How often depends on your climate and how you store the trailer. A couple of years seems average.
My trailer just "graduated" from Turtle Wax to the Meguiar's kit. Our high-altitude Arizona sun isn't kind to gelcoat.
You shouldn't need to do anything to the wood on the underside. It was treated when it was built with fiberglass resin. The real key to keeping the floor in good shape is keeping water off the top. Floor rot is almost always caused by unchecked leaks
from above: windows
, vents, plumbing, even little things like the holes where the taillight wires pass through the shell.
Better to spend your time under the trailer removing rust from the frame and treating with a rust-preventing paint