Depends on if the ribs are all welded together (steel or aluminum). If they are NOT welded, then yeah, the interior plywood is really providing structural support. It also depends on whether you want to keep the trailer from gaining unnecessary weight. (Increasing weight is hard on automatic transmissions, even in 1-ton trucks. Don't "axe" me how I know, just never drive steep roads and downshift as a form of braking. Your torque converter fins can come apart and trash the clutch cluster.)
<---each of those is a thousand bucksIf you find that the ribs are not all welded top and bottom, you could remove all the plywood and cross-brace the ribs with metal strap attached to the ribs with flathead machine screws, rivets or self-tapping screws. Then you can put foam weather stripping tape over the ribs, but not over the fasteners. It levels everything out so you don't end up with bowed, interior plywood on top of every fastener. If you're really thinking ahead and want longevity for your work, you seal both sides of the plywood and the end grain against humidity (with a non-toxic sealer) so warpage doesn't become an issue. Winter is the worst, because sleeping in it causes lots of condensation inside the walls.
Fore wat itz woyth,
Prem
My goal...
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...is to live in a trailer.