Having, in the past, a number of years of owning and working on
fiberglass boats, I'd say you may have on ongoing problem. In boat molding, and I suppose trailer molding is the same, the mold is first sprayed with gel coat. When that is cured several layers of
fiberglass cloth is laid in with polyester resin. Or, more cheaply done, chopped fiberglass strands are sprayed in with the resin (known as chopper-gun laminate). Actually, your underneath pictures look like chopper-gun. In any case, if not properly rolled/squeegeed out bubbles can be left in the laminate. Gel coat is hard and brittle and will crack if not supported, so it will eventually crack and open up over bubbles. That looks like what is happening to your trailer. Note all the little gray spots around the black holes. Probably more bubbles under the gel coat which will eventually break open. (Amongst boaties there was also a question of polyestermite infestation.
) You might want to consult with a fiberglass boat repairer/refinisher if there is one near you.