For very old and very weathered gelcoat, people seem to be getting the best results with Zep high gloss floor wax. Depending on climate and storage people are getting 1-2 years between applications, possibly more if you have indoor storage.
There are numerous old threads on the prep, application, and removal process. be sure to use the "site search/google" option at the bottom of the search menu for best results.
Before applying the first time you have to get the trailer really clean and remove all the chalky oxidized gelcoat. I would wash with Dawn soap, then Barkeepers Friend to remove the oxidation, then a final wash with clean water. When dry, wipe on multiple thin coats of the floor wax with a white cloth. For re-application, you will need to use the Zep stripper.
The first time might be a long day's work on a small trailer, but reapplication is easier.
Good call on the frame. Frame failures are common on older Casitas, and typically happen under the front of the cabin where the frame transitions to the tongue. Wouldn't hurt to have a professional welder have a look.