Hello NanciL. Just saw your post or I would have responded sooner.
My last fiver had a super slide and it too looked liked it sagged when out. What I did was to use a three foot level on the dining table to see if it was indeed sagging and the table was level. I than used the level on the outside of the slide and it showed it was plumb but it sure looked off somehow but I let it go.
After spending a winter in Arizona and checking it a few times the level showed all was good. No actual sag.
What I did notice though was when I pulled the slide in was that it contacted the side of the trailer at the top first then the bottom pulled in. I'm thinking that the slide is not really square to affect the closing at the top first thereby giving the illusion of sagging.
I would use a level to see the status of your slide and if it's really sagging then back to the dealer to have them check it.
At the CG we were at though I did see some RV'ers had slide supports under them. When I asked them about it they had no real reason they had them other than they were concerned about the possibility of the slide mechanisms bending. I never had a problem though...