Nothing that comes in a spray can (except maybe cleaner) should ever be used on the outside of your trailer. Any spray can paint
will fail within a few years and you will have a mess.
BUT, I believe that you can get custom tints both in Briteside as well as Rustoleum TopSide Marine paint
. Regular rustoleum should be left for lawn furniture, fences. etc. If you have used Rustoleum spray can primer already, at the very least, I suggest recoating with their Marine primer, if not removing it entirely. Many paints and primers just don't bond to fiberglass and/or gelcoat very well
And yes, it will take at least two coats of Briteside for a good white finish. Had the same issue when I painted my Hunter Compact-II. BTW: I hope you used Interlux "Pre-Kote", it forms a polymer bonding link of some sort with the color coat.