'Surprised no one's weighed in on this. I know a number of us with U-Haul
campers have had issues with sag in the roof---from a heavy roof-top AC unit, or a tree branch laying across it, or even heavy snow-load in winter.
Perhaps a search for "sagging roof"?? Or you could join the U-Haul
[Burros welcome FaceBook page for more discussion?
] As I recall, the first suggestion is usually to prop up the roof from inside, probably raising it a little more than usual, and letting warm sunny days "relax" the sagging f/g.
At least one member "over there" has propped the sag up, and then fiberglassed a stiffening rod to the underside of the roof---in the middle, where the ceiling carpet strip would hide it. Another is I think in the process of  temporarily removing the roof vent fan, and then  sliding pre-shaped wooden ribs from side to side in between the inner and outer skins---presumably using thickened f/g resin or some-such to bond the ribs to the skins.
'my $.02's worth..........