Mine has corrugated plastic the whole length, it's attached with a metal trim that uses self tapping screws to frame.
Couple weeks ago I noticed about 5-6' had come loose and was actually riding on top of the leaf spring shackles.
I loosened the screws in the immediate area and after trying a couple of ways to support the bottom (with boxes and coolers) I found that C clamping two 2x4's across the bottom of one side, laying a small section of plywood across those and then C clamping the 2x4's to the opposite frame to compress the bottom therefore taking up the pillowing slack giving me the material to tuck back under the trim and re-tightening to secure.
Always a challenge working alone with large pieces but this worked well for me.
Bill