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 think most of them sag a little. Pretty hard to get around it. Sticks and bricks standards for drywall, etc. is 1/360 of the span I believe. So, in 13 feet, that's about 7/16". It would have to be pretty beefy not to sag at all, then the weight would be the issue. If it's structurally sound and seals the weather out, I wouldn't worry too much about it. My .02.