We had an 11 1/2 tc for a few years with a similar issue. Part of the issue seemed to us to be the fact that both the bath and the generator were "hanging out" beyond the truck bed -- not a whole lot of support. Adding to the woes was a severely water damaged roof that chewed into the entire support structure of the unit. Perhaps without the water damage the engineering would have been sufficient to provide the needed support . . .
I rebuilt the roof, replaced a lot of lumber in the process, but of course, there was no way of getting rid of the sag. Never tried adding additional support from under the bed, however whenever the camper was on the truck and parked for use, I'd run out the rear lift jacks to give a bit more support -- and leveling for occupancy! Eventually sold the rig -- got too hard for our old bones to crawl in and out of the bed.
Sorry, missed your original posting, let us know if you tried this and what the results were.
