Removing light oxidation

Wax and a lot of hard work buffering.

Our local marine fiberglass expert (Dr. Glass) recommended this product. I tried it and it did work quite well. Read application instructions or you may have a hard time removing!

If you can't get any help as suggested by B.O. then there are products for fiberglass that will help minimize the oxidation if you don't want to paint. Many here like Maguiiars products, me included.

No white gelcoat. Trailer is beyond 1yr warranty

It's probably not painted, just gel-coat. It will only get worse as time goes by. You can rub and polish all you want. The only cure is to have it painted. See if your warranty will help out. Some will, some won't. Keep hounding them.

B.O.

Is your front cap painted brown? I had the same problem on my 2016 Crusader; brown painted front cap oxiding/fading. Just got it back from the dealer yesterday. Had the front cap stripped, repainted and re-decaled under warranty due to faulty paint from the paint manufacturer. I notice yours is a 2015. I would call the dealer/manufacturer and have them repaint it, under warranty, of course.
Also, I have been told that the only real solution is to have it stripped, repainted and clearcoated otherwise the oxidation will return.