The first thing I'd advise is that you buy the very best primer you can afford, preferably one specifically for fiberglass surfaces. I actually think this choice even more critical than the topcoat, as this is the surface to which the finish coat must adhere.(Do make sure it's compatible with whatever topcoat you settle on.)
The success of every paint job is 95% dependent on the preparation of the surface before application of the topcoat.
Wash the trailer very thoroughly, then sand lightly, taking care not to go through the original gelcoat. That's your base, and presumably still well adhered. Don't use too harsh a grit for this step- 80 grit for example will probably make scratches that will show through the paint. 120 is probably sufficient as all you really want to do is provide "tooth" for the succeeding primer. I prefer wetsanding to dry- much easier to keep rinsing dust away, though the paper is more expensive.
After sanding, wash again, then wipe down every square inch
with some kind of pre-paint-prep such as this one
. This step's important for removal of whatever traces may remain of wax/silicone/WHATEVER that P.O.'s may have put on the trailer.
This prewipe step should be done immediately before painting
to insure no contaminants settle on the trailer before you get the primer on.
Prime and paint according to directions for those products- some may require cure times between coats and/or sanding.
Nearly ALL paints require time to fully "cure" to their ultimate hardness/resistance to scratching so be prepared to treat the trailer with kid gloves for awhile after you paint it!