If it is the driver's side front slide, we use two pieces of vinyl flooring and lay them down perpendicular to the slide fairly near the middle. That big slide is unsupported in the middle and it tends to droop a bit. We had our floor replaced with wood with Ernie Ekberg and that is standard procedure. Also I believe there is a specific item made for that application on Amazon, slide slipper or something like that. We had the vinyl flooring laying around and it does the trick.
2007 Navigator, 45' Series 60
Full-Timing since 2004