New to me camper, missing door.

They basically built this frame more like a truck frame, the two main frame members are fairly stout, the cross members only tie the two together and hold up the body. The front and rear cross members hold most of the body weight, which isn't much right now. I do have some concerns about roof load though. The roof has almost no middle supports, only a few steel straps that hold up the foam head liner and insulation and the four 1" square posts holding up the edges of the top. The walls actually help support the top better than it most likely was when new. My concern would be what if we got a big snow storm? I could easily see the middle of the top bending inward. Of course, whether its up or down, the snow load would be the same. Its no doubt been outdoors since new, and survived many big snow storms, it just don't look very well equipped to handle such loads. (I would never try to walk on the roof or even lay across it to work up there). I set the new roof vent panel in place using a scaffold and planks.

The lower body sort of hangs over the top tube of the lower walls, then its secured by pan head self tapping screws all around the lower edges into the wood. Its got rubber mounts and nuts and bolts attached to plates welded to the end of each cross member.

I've got a line on some fiberglass panels which I hope to use for the new sides, I'm debating on whether or not I'll need to use plywood under the new panels or to just add more vertical 1" tubes to rivet the panels too.
I'm leaning towards laminating them onto 1/2" exterior plywood for strength.
It would still be lighter than what's there now and much cleaner looking.
I plan on making aluminum corners from .040" aluminum.