Taking apart an Airstream provides many opportunities to marvel at the strength of thin plywood.
As you can see here
there isn't any longwise support except at front and rear. The crosswise support is also quite sparse. And all that sits between this frame and you is a quarter inch thick piece of plywood and some foam. The top part of this - the support the ply was screwed to, is OEM.
The backside of this frame was originally just screwed to the wall with only two screws. The frontside was held up by the tambor door frame. That worked for 25 years but made me nervious. So I added the wall support and a stronger front frame. (you see me and you'd know why! ;-) )