For the people who suggest using epoxy or some other product as a crack filler, but don't advocate using layers of cloth on (at the very least) the back side, all that's going to happen is you'll wind up with a nice, long, ragged crack right next to the one you just repaired. You have to both fix the crack *and* spread the stress at the crack to the rest of your trailer's intact fiberglass shell.
For my part, I'd start by drilling a 1/4" hole at the end of the crack, use a dremel tool to open the crack up a bit so my repair material can get into it, then use an angle grinder with an 80-grit flap disk to *very carefully* (let's say that again: *VERY CAREFULLY*) grind through the gel coat and slightly scoop out the fiberglass below.
On the inside, I'd strip the back your interior covering to expose at least 8" of fiberglass on all sides of the crack, then wire wheel the fiberglass to clean all remaining adhesive off. (My favorite wire wheel isn't made of wire, by the way, but a Kevlar-like material called Nyalox. Dico tools make them, look for a "Nyalox wheel brush.")
I'd make a 3" wide band of medium fiberglass cloth that's 3" longer than your crack, cut so the fibers run at 45-degrees to the long axis of the band, and lay that out on a piece of wax paper.
I'd cut a second 5" wide band band of medium fiberglass mat that's an inch or two longer than the crack and lay it out on another piece of wax paper.
I'd cut a third band of fiberglass cloth, this time with the weave of the fabric, that's 2" wide and 6" long and out it on yet another piece of wax paper.
All these layers will go on the inside of your trailer.
The 3" wide band cut at a 45-degree angle goes in first. It should overlap the end and sides of your crack by an inch or more on each side. Don't get too stressed if it becomes more narrow as you handle it; that always happens, and it only has to extend 1" beyond the crack on either side.
The fiberglass mat layer goes up next. It should overlap the sides of the cloth repair by an inch (or so) on each side.
The 2" band of fiberglass cut with the weave of the cloth goes on last. It strengthens the corner of your window so the crack (hopefully) won't re-appear. Apply this layer at a 90-degree angle to the crack to make a "T" shape patch, with the top of the "T" at the corner of your window opening.
How you apply fiberglass:
This is where I'd suggest you do things differently from the way I do. I'd simply mix up enough resin to put all three of these layers up all at once. It's stonger that way, but not all that much stronger and I've done this before, so I'm fast. Since you haven't, I'd apply these layers one at a time.
You'll need several inexpensive, disposable 1" wide "chip" brushes (my cheap source for them is Harbor Freight) and several Dixie cups. You'll also need a workspace with lots of newspaper down around it, especially where you plan to pour and mix the resin. You'll also need latex gloves, a baseball cap (to keep drips of resin out of your eyes and hair while you work overhead), and safety glasses.
Pour about 2" of resin into your dixie Dixie cup and mix according to the manufacturer's instructions. (My preferred brand is Tap Plastics Bond Coat Polyester Resin. You need to buy the catalyst separately and at the same time. Tap also sells the fiberglass cloth. An alternative source for fiberglass cloth and resin is a boat shop.)
For each layer:
1) Mix the resin.
2) Wait 2 minutes.
3) Dab the resin on the cloth or mat while it's sitting on the wax paper. Brushing will distort the cloth, so if you do brush to sread the resin out, brush lightly. The cloth or mat should be saturated enough that it's no longer white or white-ish, but resin colored. It should not be drippy-wet.
4) Brush resin onto the inside surface of your shell around the crack. Cover enough area for the fiberglass patch you're going to apply, with some overlap.
5) Take the layer you're applying into the trailer on the wax paper backing. Push the patch into place while it's still on the wax paper, then carefully peel the wax paper back and use your chip brush to smooth the patch out and remove any wrinkles or air bubbles.
6) Let it sit for an hour. Longer if its below 75F outside. Do not apply fiberglass at temps below 60 unless you have heat lamps to warm the repair while it sets.
7) Repeat for each layer. Your patch will be stronger if you apply each layer closer together.
8) When you're done with the fiberglassing, use a reciprocating "Saber" saw to trim the patch back to match your window opening.
This patch should be, all by itself, plenty strong enough to resist the crack from forming again, but you need to tie the repair structure inside your shell to the outside of your shell. To do that I'd use Bondo Glass.
Bondo Glass is polyester resin with glass fibers mixed in. It's not as strong as fiberglass mat, but it'll fill in any voids in the crack and is plenty strong enough to tie the top half of the repaired area with the strength layer you just put inside.
For your outside repair you'll need Bondo Glass, Bondo Body Filler, a tube of spot putty, some flexible plastic putty knives you can ruin, and some Bondo (or other brand) spatulas for spreading the resin out around the repair. I like to mix the Bondo on a sheet of wax paper.
First, physically rehearse the job of putting the Bondo out on the crack. It sounds corny, but it's the best way to make sure you have all the tools you need at the ready and know what you're going to do during the 5 minute working time of the Bondo. If you do this, you won't suddenly discover you forgot to get a ladder (or other tool) or find the ladder is placed in a way that makes it hard to work on the crack.
Mix a golf-ball sized wad of Bondo Glass according to the instructions. Lay out a globby snake of Bondo Glass so there's a roughly 3/8" snake of the stuff overlying the crack. Push it into the crack, then spread it out from the crack by moving a spreader at 90-degree angle across the crack. This spreads and pulls the glass fibers in the filler across the crack and helps tie each side of the repair together. Once you have a 1" wide (or so) repair, smooth the patch out from one end to the other with a single stroke or two. You will not get a smooth finish with this layer, so don't sweat it of it looks a little rough.
Come back in a few hours and sand the area smooth (120 grit sandpaper) to match the contour of your trailer shell. Wipe the area clean with acetone, and apply a smooth top layer of Bondo Body Filler.
You may need to rinse and repeat the last step a couple times before you're happy with the result. When it's really close to perfect, start using 180 or 220 grit sand paper. When it's really, really close use red Spot Filler to address any minor imperfections and sand with 220 grit paper.
Clean and wipe the area down with acetone and paint
to match. I won't go into how to paint a trailer -- that's a whole topic all by itself -- but you do have to paint the area. Sunlight will damage and crack the polyester resin and Bondo if you don't, forcing you to do the whole thing all over again.