How to remove oxidation before waxing?

Try spraying a small spot with WD40. Wait a minute and wipe with a soft cloth. See if that works.

It works wonderful for road tar, tree sap, and black streaks!

Have you ever heard of the Zep/Red Max Pro Application Process?
It works great on older fiberglass.
If you don't want to use the floor polish, just use the wax of your choice after cleaning the fiberglass.

Zep/Red Max Pro Application Process

Don't forget the before and after pictures. :)

I've used Presta Gel Coat compound with a wool pad and a DA sander/polisher. Followed that with 3M Finesse-It 2 Finishing Material using a foam pad and DA.
Last step was Collinite No.845 with a foam pad and DA.

Came out beautiful. Problem is my front caps gel coat is defective as is a lot of darker caps built in the last couple years and all that work quickly disappeared weeks latter. Hey, it was fun while it lasted.

Being that yours is older the gel coat should be fine as far as bringing the shine back and lasting.

wash your trailer with awesome(this will make it easier to deoxidation)
meguiars power cut compound #M91(for deoxidation)(orange pad)
nu-finsh(white pad)after the first app I put a second one on 2-3 week after
I use a portal cable DA polisher with lake country pads
my trailer will shine for about a year

Thank you for posting this specific question.
I make this reply so that I will be tagged on subsequent replies.