Your explanation of the "sagging" and "in pieces" sounds to me like water damage. The first thing to do is locate the water leak source and seal it with a quality sealant like Eternabond tape. Do not use silicone-containing sealants. Your thinking to reinforce that panel sounds reasonable. Silicone and RV's do not play well together, and it takes a special silicone removal product to remove and residue before anything else will stick there again.
Adding insulation is a good thing, not difficult, just time-consuming. While you have the paneling removed, is a great time to inspect the wiring for fraying, wear spots, loose connections, etc
Adding a 50A cord is just part of an electrical upgrade, the AC breaker panel should also be upgraded to the proper size. That's about all my thoughts,
Sign-in often and join in the various discussions, we will be looking forward to your future posts.