Molding fiberglass in big sheets can be hard to keep flat unless it's well supported from either side. Most boats, camper caps etc use a mold which shapes the outside of the part. This means building an original, then making a mold of that, then using the mold to make more of whatever. That's great for series production but pricey and difficult for the average guy.
The simplest way to make a 'one off' piece is to make a form out of plastic foam, cover it with fiberglass, then use acetone to dissolve the foam. This is called the 'lost foam' method. It does involve having to do a fair amount of sanding on the outside to make it smooth and it should be painted.
You build a wooden 'buck', glue foam to it, then carve the foam to the shape you want. Then you cover this with resin and mat to the thickness you want. My buddy's Hi Point trailer was about 3/16" thick, BTW. It's possible to add a layer of aluminum screen in the mat and resin to add even more strength. Then the completed piece is flipped over, the 'buck' is pulled out and the remaining foam is removed with acetone. At this point, the inside can be reinforced with aluminum tubing for added rigidity, you bend it to shape, lay it on the inside and then use mat and resin to make it a permanent part of the shell.
Here's a page on doing a small part, it works the same way on bigger stuff.
http://www.instructables.com/id/SM93MJWF2H0OXCZ/?comments=all Like the page says, you use acetone, not kerosene, to dissolve the foam.
The big PITA with this is making sure the 'glass doesn't crack when the trailer flexes. Thinner 'glass flexes better than thick. If you plan your trailer frame carefully you can minimize flex.
