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...
The angle would be installed on the inside top edge of the slide. This is where it is sagging.The 1/2 sag is enough that the middle of the slide seal does not seal very well when the slide is open.