I had bad sagging in the two front corners of my beachcomber
so I had supports welded to the frame at the front. My shell was completely off the frame so it was easy to do at that time. And I used that new support as another spot to bolt the shell to the frame for extra support. (prior to the reno I noticed the shell was bouncing a bit on the frame while towing so this extra bolt point should help give it a more solid connection to the frame).
I also had some minor sagging at the back corners but they straightened out after the shell was sitting on my driveway for a couple months while I was working on the frame. Looking back I should have welded a couple supports at the back also but I think it will be fine. The back doesn't have as large of a distance from the frame to the edge of the shell as in the front.