As always, thanks to all for replies. My wife and I spent all day cleaning out and gutting our new-to-us '69 overlander, trying to eradicate a horrid smell that we now think is a combination of the ancient holding tank and the smoker's residue sticking to everything. We have about 2/3 of the cabinetry out of the trailer, not bad for the first day!
One thing we did manage to find (behind the fridge) was the missing key! It works for the back panel but not for the front one, as sugarfoot mentions with his. However, I can get to the front one from the other side now, so all is well, at least with that problem.
The next issue to do with locks is the door. It is structurally compromised---the bottom half, below the hinge, swings fairly loosely and looks like it is only attached to the top by the skins. The break is at the striker plate, perhaps it was broken into at some point. Obviously doesn't seal against the weatherstripping. I plan to drill out the rivets for the interior skin and see what I can do to stiffen it in the correct position---perhaps bolting (or welding) in some tubular or angular aluminum or steel. Security won't be a problem where we plan to keep it, but I would like it to be weatherproof.
Thanks again, and I'll update my profile now