[Richard]

If this were me, I would remove the valence while the slide was in. Then while deploying the slide, stop it about two inches shy of full extension. That would give you a more complete picture of what is going on. Be careful not to trip on the floor which will not be level at that point.

Also while you have the valence off, and the slide in, stretch mason's twine from corner to corner across the top. Then you can measure just how much the sag is in the middle.