You've got quite a job ahead of you. I have owned an old corvette for about 35 years and a large sailboat for 24 years. I've done work on both. I see that Lowes carries Fiberglass sheets at $65 for a 4 x 8. it's about .09" thick but you could double it. Wipe it with acetone, hit it with 100 grit paper and glass it together to fill large areas. You can buy glass matt and roving to fill and bond with. USC sells a fiberglass filler called Dura-glass that works well. It can be bought at an auto store that specializes in paint
supplies. West marine sells West System fillers that can be added to epoxy that does the same thing. To fill a hole you must cut a piece of stock to fill it and grind both surfaces back about 4 times the thickness. Secure the piece with tape from behind. Cut your glass mat to size. Wet the surface and apply the mat. Brush more mixed fiberglass on with cheap, throw away brushes. Build it up with successive layers. Remove the tape and bond on the inside. Cracks can be ground open. a Dremel works well. The duraglass works well for filling as does epoxy with filler added. Epoxy and fiberglass work well together. Epoxy will not accept water, Fiberglass is more porous. When your done with the glass work you will probably want to hit the whole trailer with a couple of coats of epoxy primer and follow up with a good 2 part marine paint
like Interlux or All Grip. There are tons of videos on YouTube that can be a lot of help if you haven't done much of this sort of thing before. I often watch the videos anyway, easier is better. Good Luck !