I should also that when you cover with new plywood, glue it down with a good construction adhesive like PL Max (get it one of the big box home stores). It's very strong yet flexible and will make the new plywood part of the structural panel. This way you only need a few staples to hold it while the PL dries.
The Lance that I rebuilt (link in my signature) had an aluminum roof, that's another option if you can get a roll of aluminum wide enough to cover with no seams. The aluminum would be sturdier than the rubber but much harder to work with outside of a factory setting.
