“Lots of cracks” sounds possibly like spider cracks in the gelcoat layer, which are non-structural and cosmetic. It is not necessary to do anything but keep a good coat of wax on the trailer.
They sometimes result when the shell flexes because the gelcoat layer is more brittle than the underlying structural fiberglass. If there are a lot of them, it might be worth investigating whether there is a reason for the excessive flex. Could be a loss of interior support or a harsh ride due to an aged-out torsion axle
Gelcoat repair is possible but could get quite expensive if a lot of area is involved. Paint
is another option. It depends on budget, time, and project goals. Are you trying to restore to like-new or just make it clean and functional?
If these are deep structural cracks, they they will require fiberglass repair, usually from both sides. Again, the cause should be addressed.