Donna>>I also travel with the door locked.
Me, too, Donna. In fact, the only time I lock the door on our trailer is when I'm pulling it down the road.
One of my first 16 footers had a door that used to pop open, in transit, occasionally, but usually because I'd forgotten to lock the door ... or as part of breaking camp, give a little tug on the door to make sure it actually secured.
If I remembered to lock the door ... and double check that I'd actually shut the door tightly ... it didn't happen.
However ... any trailer will flex ... particularly if you pull them down the kinds of roads I do! In the winter ... some of our pot-holes are the size of Texas! You hit one of those just right and your fillings will fall
out of your mouth!)
My current rig ... a 17 foot Casita
... has a dead bolt that prevents the door from opening ... provided you lock the dead bolt. I only use the dead bolt lock ... and have never locked the "regular" lock.
Now, as I said. The only time I lock the trailer is when we're towing it.
As most of us know, any trailer door can be easily "popped" open with a crowbar. Since we don't keep anything of value in the trailer (I try to remember to take our jewels with me ... when we go hiking, etc) ... I'd rather someone enter our rig and take a look-see rather than mess up my door frame with a pry bar.