Update: the floor is finally level and strong!
since it became apparent the slideout floor would never bend back into position, which eliminated trying to attach a new piece of plywood and pulling the sag up or otherwise, we went with buying 16 gauge 1” square metal tubing and had 14 pieces cut to 40”. Got them pretty cheap at https://www.onlinemetals.com/en/buy/carbon-steel/1-x-0-065-carbon-steel-square-tube-a500-a513-hot-rolled/pid/10301
we attached them 12” apart to 3/4” sande plywood which wasn’t cheap at Home Depot but the cut it to 4x 40.5 inches for us. We intended to glue them so we could flip the board and they would hold in place to screw from the top, but we found this fantastic glue, Fuze It by Liquid Nails, and it stuck so well we decided we didn’t need to scree them also. Here is the glue; https://www.homedepot.com/p/Liquid-Nails-Fuze-It-9-oz-Gray-All-Surface-Construction-Adhesive-LN-2000/206736831
we used 8’ and 4’ pieces of plywood and fit them together Now we have a level floor All that is left is to carpet it
Pictures To follow