I think I'd fall
back on the way Gina D was fixing up her Leocraft while she was in Cali. However, FIRST I'd get the PlastiKote Fibered roof coating OFF the roof of the trailer, and then I'd look into why it is on there in the first place - are there cracks in the fiberglass? leaks
Then find a way to deal with those issues - but another coating on top of a coating that isn't working out real well - somehow just doesn't seem like a great plan. I'm NOT a fiberglass expert by any means - that's one of the many reasons I that read virtually every message on this site - I need to learn - a lot!
From the description, "the Plas-t-coating on the penetrations has small cracks forming" makes me wonder if it isn't just leaks
around fan openings, fridge
openings, sun roof opening, antenna openings, vent openings and etc. that were patched with the "plasti-kote"
If THAT is the case, the answer is to remove the vent or what ever, clean up the opening and reinstall with Butyl rubber tape (NOT SILICONE!!!!) and clean around the area., getting rid of the "foreign material" which likely won't adhere well to the fiberglass gelcoat anyway.
Unless of course, I'm completely wrong. (I was wrong about something once - I thought I'd made a mistake, but I really should have known better than that!