This may not be what you want to hear, but fibreglass work is not so bad. A cheap bunny suit, gloves, some fibreglass mat and some resin and matching paint
. As well as some sand paper, a sanding block, maybe a cheap, (less than $40) grinder, some disposable paint
brushes and time are all you need.
I find it very therapeutic. Grind it off. If you grind too much, than lay some more down. If you lay too much down, than grind it off... You get the picture.
Seriously, the approach I would use to close a large opening is:
1) Grind the edges of the hole that you are covering from full thickness to paper thin over a distance of 1-½” to 2” from the edge of the hole. Typically from both the outside, and inside.
2) From the inside, screw in a piece of ¼” plywood that is 2” larger than the hole on all sides. It should be covered by a sheet of poly to prevent the fibreglass from sticking to it. Yes, I mean you should screw through the shell, in as many places as required to make the plywood conform to the curvature of the shell. Drill a hole through the fibreglass and plywood that is slightly larger than your screw. Use a large washer under the head of the pan head screw, and a block of wood, on the other side of the shell, to screw into, or use machine screws with large washers on either side of the fibreglass to spread out the force, and a nut instead of a block of wood. Start in the middle of the curve you are trying to follow, on either side of the hole. Drill subsequent holes only after the first two are secure. That way following the curve will be easier.
3) From the outside, glass over the plywood to the same thickness as the shell. The surface area provided by grinding 1-½” to 2” around the hole is what the patch will stick to.
4) When it sets, remove all fasteners and the plywood sheet.
5) Glass the screw holes. I would stuff some resin soaked glass in the holes and flare out the plug around the hole, both inside and outside the trailer.
6) From the inside, glass over the seam and the screw holes, (after sanding down the plug).
7) Sand and Bondo, (an iterative process) the outside of the new patch, till you are happy with how smooth it is.
OK, not super easy, but you will be glad to have these skills later.