This is Bad. The travel trailer is essentially extra rooms for our house, and we use it regularly. I put on my coat and joined Lisa to examine the issues. The door wasn't locked. What happened was the same thing that happened many years ago up in Mehama: the tang that engages the outside handle to the latch bolt (the part that pulls back into the door when you pull the handle) had broken.
There is an way to get into the trailer other than by the door, but it's messy and a tight squeeze: If you take everything out one of the storage compartments, it's possible (for Lisa, not for me; I'm too big) to climb through a hatch in the floor. However, that does require unlocking the storage compartment, and both keys to the compartment were inside the trailer!
Lisa got tools and examined the lock. After much experimentation, she managed to get a thin screwdriver in behind the outer handle and into the area where the tang used to be and just barely engage the latch bolt. That wasn't quite enough to get it open, but while holding that with one hand, she used a set of pliers to gently pry on the bottom of the door frame just enough to make enough clearance to get the door popped open.
So with the most immediate problem solved (and Kuma Bear rescued from where he was sitting on the couch inside), the next job was repairing the lock. Fortunately, when this lock failed years ago, we bought two replacement door mechanisms. Getting out the keys to the storage compartments (and giving me one to go store with other spare keys in the house in case we ever need to access the hatch this way again), she got out the box that contained the second replacement. She then disassembled the broken latch assembly, removed it, and set it aside.
Putting the replacement assembly into the door proved to be more challenging than expected. Many years of use has led to the door being not quite as in true as it once was. Lisa was obliged to use gaffer tape to shim the assembly so that it would fit tightly and hold the door correctly. After multiple times, she was ready to give it the acid test. She went inside the trailer (opening from the inside has always worked, even when the outside tang broke) closed the door, and reopened it. She then closed it and I opened it from the outside. We tested the keys that came with it. They were stiff, but after Lisa lubricated the assembly, they worked fine. It took an hour or so, but we once again have a working travel trailer door.
The company that sold those door latch assemblies no longer makes them. Also, Kit Manufacturing (the company that made the trailer) ceased operations years ago, after a more-than-fifty-year run. The door latch is a pretty standard part, though, and we found others online. Lisa is concerned that what are already not-that-great assemblies made from "pot metal" will be even worse from "generic" replacements. We ordered one. If it seems serviceable enough, we'll probably buy another. With some parts, we feel obliged to always keep spares on hand because if they fail, you need a replacement now, not two weeks from now or maybe never if the manufacturer says, "We don't make that anymore."