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...
I agree, a 1/2" sag over a 12-14' span is nothing. I'd leave it, there are different (hundreds ) of slide seal styles out there. If 1/2" affects your seals have them swapped out or install a slide topper.