First of all there is a huge difference in paints and a seals, or Gel coats,
fiberglass appears to be a solid surface, it is actually a coated wick with a smooth outer covering a few mils thick, (a gel coat) a crack in the outer gel coat allows moisture to seep and wick straight thru the shell...I use Interlux products or West system myself...There is a self leveling prime coat repair made today, its a two part epoxy called "Primecoat"...being a two part epoxy it will fill cracks, its designed to restore old gel coat finishes...also its brushable! As long as you follow the instructions and first wash the surface with the recommended solvent needed...Then I would paint with "Perfection" another Interlux product and is brushable...you will end up replacing the gel coating with results that are factory in looks...These I recommend as they are UV stabilized products and have served me well.
Fiberglass deteriorates because its plastic, it out gases at a set rate, you could park it in the dark and in twenty years it will look aged and be cracking, the sun on older trailers greatly accelerates deterioration, the outer covering is the thickness of a heavy trash bag...The good news is new chemistries today have made this a far easier job than 20 years ago, the plastics today are more stabilized and a whole lot better than paints...Some old timers will say fear this job and go to a shop...I agreed for the most part with this up to a few years ago...I would research my suggestion here and then see how intrepid you are...I did it and loved the results...Other options are single part epoxies or silicon urathanes which are easier...some now are water based and clean up with water.
The non specialized paints I do not recommend, they do not make good long term sealants for fiberglass issues and are hell to remove when you must do it right later again.
Harry