Your problem is most likely is due to the breakdown of the wood framework that is glassed into the door the door. It is especially weak (at least on my mid 80's Casita) at the center where the door latch mechanism is mounted.
On my trailer a PO added a lightweight cable anchored near the top and bottom of the door with a turnbuckle placed in the center. The door curve can be adjusted by turning the turnbuckle in or out. It's a little hokie but the set up works fair but not perfect.
The best way to correct the problem is to take out the old rotten wood and replacing it with new. This will require some fiberglass work. That is what I plan to do on my trailer eventually. There is a guy named Eric who rebuilt the door frame on his older Casita
. He posted a lot of pictures. Here is a link to his post.
I think he had to do the job over a second time to get it right so make sure read all of the way through his posting until you to get the second repair.