Just to add to Pete's description, I've done a diagram of one way you can make this repair. Applying fiberglass and resin isn't too difficult, but keeping it off everything else is a bigger problem and knowing how to set the job up is hardest of all.
I hope diagram 1 shows your current problem in cross section. Remove the window and chamfer the edge of the existing shell where you will be building out as this makes the repair stronger.
2. Fit a temporary flange to the outside of the shell with a smooth side facing inwards - double-sided sticky tape is good for this (it stops resin from seeping between the existing shell and the temporary flange) though back it up with some duct tape elsewhere as some sticky tape is softened by resin. The flange must either be made from a plastic that fiberglass doesn't stick to, or it must be coated with mold release wax (6-8 coats) before it is fixed in place.
3. Laminate a repair strip onto the edge of the window opening - carry over onto the existing shell by at least an inch and go out past the point where the opening will be cut.
4. After the fiberglass has hardened, remove the temporary flange and trim the opening to fit the window.
All of this is worth trying out using a scrap piece of plywood to simulate your trailer shell, before you try it for real on your trailer.