We have a 2007 Springdale 290CT with a 13' slide that has about 1/2" of sag in the middle. I talked to a local rv repair shop and they said their repair would be to remove the trim on the inside, jack...
How is an angle brace on outside area of roof going to remove a sag.....?
Sagging in middle would require a brace across middle where the sag is.
Even if sandwich design the pieces of sandwich should be solid (no overlapping seams between top pieces and bottom pieces)
Other than water intrusion to weaken material OR seams lined up causing a weakened area...roof shouldn't sag. The bracing just doesn't sound like the correct repair...to me
Guess Repair shop has experience with this type of problem