My Burro does not lock from the inside either. The small balck latch seems to want to move up and down but does not. If you watch it while using the key from the outside, you will see that is exactly what it does. I've tried my standard "first response" of a shot of WD-40, but that didn't help. I firmed up all the screws and that didn't work. I thought about taking it apart, but was afraid I would make it worse and it wouldn't lock from the outside!!
We use "second response" - bungee cords!! I put one end around the door handle and the other on a knob around the corner on the inside!! I figure if anyone tries the door - we would at least be alerted and they would be startled by a loud thwack. Hasn't kept me from sleeping.
Per,Gina & Christie: Thank you for your suggestions. Per, your picture turned out just fine. My looks similar, but without the deadbolt. Gina, my door is flat also, and does lock fine from the outside even with the door not level. Christie, your reply is exactly what my door latch system is like. I will probably have to try your bungie cord idea and hook the other end to the toilet seat!!!
I also added more RV info to the profile area under my name.