That crack looks to me like it was caused by weight
pressing down on the roof. Maybe snow.
If I were going to fix it, I would remove the drip cap from the outside and the Ensolite from the inside (extending out about 8" from the crack, or as much as you need to be able to work comfortably).
Next I would put in some temporary support and arrange it such that the gap was closed and the trailer was back to its original shape (watch out that you don't "overjack" and cause problems somewhere else).
Then I would clean the inside surface of the fiberglass (inside the trailer) with a mold-release wax, like Interlux Solvent Wash 202 (mold release wax should only be on the gelcoat, but I wouldn't take any chances myself).
Next I would sand an area extending 6" or so beyond the crack in all directions. You will probably have to bring a bit of glass around to the outside of the trailer where the crack hits the door jamb - I can't see it closely enough, but you may be able to keep it confined to the "lip" that the door covers up. You can use a fairly rough grit for this. What you want to do is make it reasonably flat, and knock off any shine. Watch that you don't go too deep. Also be sure to wear proper protective and safety gear.
Next I would cut some patches out of fiberglass cloth. I would probably use something like biax/mat 1708, but you don't need anything that heavy, really. With that one or two layers would be way, way stronger than the original trailer (these are actually built rather weakly, which is why they don't stand up well to things like snow load). You can make one patch slightly smaller than the other one to stagger the edge (it will be covered by Ensolite, but still a good idea to prevent a bulge and/or a stress riser). You can feel free to mark up the patches with Sharpie for orientation, etc., as it won't show later.
Note that I am totally switching from "I" to "you" here
Now mask off everything except the patch area. Epoxy WILL drip
You can now lay out the dry patches, right on top of each other like they will be when they are on the trailer. Thick plastic over cardboard makes a good work surface, or a dishpan, or etc. Mix up some neat epoxy resin and saturate the patches with it. The white cloth should go clear. You can use a squeegee or an old paintbrush. Paint
some of the resin onto the camper where you plan to patch. Remove excess resin from the patches, and then lift them as a unit and position them on the trailer. You are going to be working against gravity, but they will have a certain stickiness to them. You can hold them up until they cure with blue tape, or some people make a form (flat something with plastic or waxed paper on it) and then hold it up under the patches with a stick), but that might be a bit tricky with your concave area. You can work some of this out ahead of time with the dry patches. Also, like stir-fry cooking, have all the "ingredients" ready and set out ahead of time. For example, if you are going to use tape to hold up the wet patches, cut off lengths ahead of time and have them hanging somewhere, ready to grab.
If you have a rough or overlong edge at the door opening (I can't see whether you will be wrapping it around, but you very well may be able to just run it flush with the door edge), you can just let it "run out" wild; then, when the epoxy gets to the "green" stage (can still dent with thumbnail), you can easily cut it off.
Once it's cured, you wash the surface with plain water and a Scotch Brite to remove any amine blush (waxy coating that can form), and then you are ready to sand off anything that you feel would show through the Ensolite.
Then glue the Ensolite back, and know that that part of your trailer is stronger than all the rest
PS: Speaking of messy epoxy, one trick I like to use is to put two or three of the blue nitrile disposable gloves on each hand; then, as they get stick, I just peel them off to reveal a clean glove beneath. That saves a lot of "oops, there went a glob of epoxy onto my wrist" while you are trying to dig out and don a new glove.
PPS: This was just a basic outline, with only one small photo to go by; so feel free to post more photos of the crack, or ask more questions.