how to fix this mess

A rear jack on my Bigfoot was similarly damaged although not nearly as bad as yours. The bottom was pulled out about an inch. I used glue and some serrated metal strips designed for just such a repair to reinforce the holes that were stripped. I found the two lowest screws are in just the thin fiberglass shell; there is no solid backing behind these. It took a long time to scrape off the sealant they used under and around the jack braket. I opted to reuse the original screws rather than go up to the next size and it appears to be pretty solid. But you may need to upsize the screws with the extent of damage you have. It helps that the force on the screws is a shear force rather than an axial force. And that the rear jacks carry much less of the load than the front jacks which is likely why they don't wrap around underneath the camper like the front jacks do. Good luck with your repairs.