That's not true. You have to be able to open the door in order to raise the top. It is no harder to unlock/break the lock on the door with the top down or up. The only thing having the top down does is it forces you to crawl around inside if you DO get in.
The door locks on the HiLo are relatively simple and probably easily broken into or picked. And, someone with a pry bar could probably break into the door using it too.
You've had good advice regarding removal of the battery, propane tanks, tires and wheels (including spare) and any other item of value from the outside or inside of the trailer.
- Jack