Folks, if your locking pin corrodes and jams in place when the door is locked, eg, during a long storage period, you will not
get the latch open. So if you're storing your vintage Airstream and not moving the lock now and then, beware. We all probably ought to open up the skin a bit and lubricate our old Bargmans today
Here's what the body of the lock looks like from the back side. (The keyed lock actuating barrel is missing in this particular spare part.) Suffice to say, the rotation of the key turns a disc at the end of the barrel. This disk has a pawl on it that moves the locking pin up and down about 3/32" of an inch--very small movement, but it puts the end of the lock pin into the body of the latch so that it can't be moved. The rotating disk on the lock actuating barrel does not look overly strong--I'm sure you'll break the lock if you try to turn it with very much force. I almost made the bad decision to force it with a screw driver--this would not have worked.
Here's how to fix it if it jams in the unlocked position:
Drill out enough pop rivets in the upper door skin to lift the skin up far enough to get access to the lock. This requires removal of rivets up almost 2/3 of the skin height.
When get access, you will see that the lock is covered by a hat ("U") channel. You have to cut away a portion of this bracing channel in order to get to the lock pin.
The lock pin has a cutout on the face away from you, which is difficult to get at. Use penetrating oil and then whatever tool you can to tap it up and down until it is freed up. The orientation of that flat cutout is maintained by the manual lock pin/bar/thingy that protrudes on the inside of the door. I would not hammer on that pin too hard. It might temporarily solve your problem, but you'll still want to open the skin up enough to lubricate the locking pin.
Fortunately this happened in the back yard and the door wasn't locked.