I just finished removing the 4 sheets if 3/4" plywood plus thin rubber mat out of the flatbed and replaced it with a 2" XPS foamular layer plus 3/4" horse stall mat from stockyard ranch supply. I shaved another 1/2" or so, lost weight and gained insulative benefit. Since my box is square and something like 62" wide, I ran them sideways using two sheets cut. The horse stall mats are designed to prevent horses digging and is unbelievably strong and heavy; heavier tha same thickness of plywood. When taking out the old plywood I found the bolts very rusty, a lot of moisture trapped by the plywood and the bed was rusted. The plywood was a little moldy. On the othe hand, keep in mind this plywood has ridden under the camper nearly 100% of the time for 110k miles and from Maryland to Argentina and back. So it worked well but needed updating. I took out the 4x6 x 8' camper guides as well.
After removing all that stuff I wire brushed the stew diamond plate, painted it with rattle cans, put in the XPS the matts, put a mat on the front wall/headache rack, cut some new guides and laid them in the bed. They actually open up wider at the rear to make it easier to get lined up and into place and get narrower at the front. Finally loaded the camper back up just before the snow hit last night. Today I found I could gently tap another shim piece in between the guides (not even bolted down) and the boxes, effectively wedging the guides against the camper. I think it'll be both easier to load AND less able to shift around. Very pleased with it! We are camping in it tonight at 16 degrees outside, toasty warm inside!
I think with the super strong horse matts over the good XPS running sideways exceeding the TC width I think it should hold up well. The plywood added to trapped moisture as the XPS probably will as well but at least the XPS won't soak it up and hold it against the steel. I am really glad to be done with the plywood but it did serve its purpose for 7 years and and about 750 nights of use. :)