Five times now I have had clients call me unable to get into their trailers due to a serious issue with their lockset. The knob just spins and spins not allowing access to the trailer. When this occurs you will have to gain entry through a window and open it from the inside. Today, it happened to me on my trailer. The L100/200 was manufactured by Bargman Co and used in almost all models of Airstream 1961 through 1968. Both lock sets use the same exterior knob. The L100 has the same one on the interior too.
The L200 uses this knob on the backside. Up opens, down locks it. These locks were discontinued about two years ago. When still manufactured they cost about $85. Now they commonly sell for $250. I saw one sell on Ebay for $450 just recently. So, if the outside handle just turns and turns, you need to take off the back side. Just remove those two screws you see and the back comes right off.
In the very center is a philips head screw. Tighten it. Yes, it is that simple. If that screw loosens just a little bit, the lock fails. I suggest before yours causes issues, you remove the screw, coat it with LockTite, and put it back in.
I just saved you a pile of money. If you want to show your gratitude, feel free to send it my way....