Last year coming home from a desert trip in our toy hauler I noticed that "the drunk handle" (as some call them in our camp) from our rear door was extended while we were on the highway.
I pulled over and shut it and assumed one of us forgot to close it before we left camp. It bugged me though, because I always do a good walk around before we leave camp: look at all cargo holds, inspect all six wheels/tires, check TV antenna, and definitely check the drunk handles on both doors.
As I was ruminating over this (back on the road), I saw the handle open again.
It turned out that some of my cargo in the garage (folding chairs) was pushing on the door, actually unlatching the door, and opening the drunk handle.
From this I surmised that it really doesn't take much pressure on the door to push that handle open.
And now I deadbolt that rear door when we hit the road. No more troubles with the drunk handle.
My experience isn't very scientific, but I still consider this myth to be busted. Just push on the door with the same weight and pressure as a camp chair.