My Crown has a ring welded under the front step. The door key that opens the side compartments also has a ring. There is a hole in the front step that the door key slides up into and it goes into the hallow channel on the door frame to keep it locked. This was the standard locking mechanism on all Crowns from the 50's until the end of production in the early 90's.
When I'm inside for the night, I have a wooden brace I place between the door and the first step to prevent the door from opening. The door also has a release mechanism from the inside which will allow the door to open outward in a emergency. I also have two rear exits I can use to get out quickly if needed.