Ouch! Bummer dude!
I agree, taking the under skin off and supporting the camper without using the rear jack(s) will be necessary.
Presuming it will sit on 4 jacks now without the truck under it based on your first repair?
If so, I’d unload it from truck, get it sitting on the jacks and remove the whole skin underneath. I’d think it better to leave it up at truck loading height or at least far enough off the ground to work under it.
Then build a frame to support the back of the camper, placed as far back as you can behind the center of gravity while allowing as much room behind the support to work on it.
Then tear into it.
If you get lucky, it may have just ripped the bottom corner connection of the frame apart and might be able to repair it from underneath?
On these campers, it’s 90% the lower sides that support the weight of the camper in the truck. Like 2 joists front to back one on each side. In the middle is the tanks, and just some bracing mainly holding the tanks up and in shape.
Good luck!!
