I would do some reading before you get too far. Near the top of the screen on this site to the right you will see a tab: "Manufacturers". Click on this tab, then select Trillium
. You will find hundreds of threads on Trillium, lots of them concerning repairs.
Some common issues on Trilliums are rotten wood where the windows
mount, leaking belly band, door sag from hinge wood rot (hinges are screwed into wood blocks inside the fiberglass), body to frame bolts (always rotted out).
Unless you see clear signs that these repairs have been done, you should expect to face them in the near future.
Love my Trillium, its an awesome trailer, but there were some design flaws. And compared to other 40 year old trailers, the issues are quite minor.
Other issues common to all vintage trailers include worn out axles. These original axles were designed to last 15 to 20 years. At the 41 year point, they tend to be well past an useful life. There are threads on this topic too.
Consider most of your issues others have faced in the past. So with some reading you will find hundreds of approaches to addressing these problems.
Removing rivets is easy. You use a drill to remove the head on the rivet. Drill sized should be based on the shank on the rivet, not the diameter of the head. When reinstalling stuff inside, you either replace the rivet, or place a small block of wood behind the fiberglass and use a wood screw. I've done both.
There is a close to 80 page thread on polishing Trilliums titled "Not Poliglow". Choice of method and material to polish a vintage FG trailer is like getting agreement on religion or politics. Not going to happen. I am in the Zep floor polish camp.
The one thing I did wrong is I polished the trailer BEFORE fixing the belly band and the door. Mistake! Needs to done AFTER all other outside work.
1977 Trillium Outside Clean and Polish
, on Flickr