I had this happen with our Scamp
. Over several years the gap at the bottom of the door increased to the point that I couldn't trust the original equipment latch to hold the door shut while enroute. I "fixed" the symptom of the problem by installing a dead-bolt house type door lock.
The real cause was revealed when I didn't slow down enough at a railroad crossing on a rural road in Kansas. The tongue of the trailer was bent up at about 20 degrees. Fortunately we were near a small town with a lumber/hardware store. I bought some timbers and ratcheting tie-down straps and made a sub-frame to correct the problem enough to complete our trip. I also noted the door wouldn't even latch now, and barely latch with the dead bolt.
Once home a welder friend and I jacked and pulled the frame back close to it's original shape and added reinforcing where it should have been stronger in the first place. Now the door has a constant gap dimension top-to-bottom and the original door latch works beautifully.
So, if you got lost in all my story telling above, if I had your trailer, I would check the frame.