The door latch lock really is only good for dropping the trailer off for service (since dealers have a master key for that lock only). I only use the deadbolt normally. Both for the lockout reason, and because it actually secures the door in transit. I've had a door come open while driving (actually more than once), because the normal road motions can cause the regular latch to "walk" open.
Recently, I upgraded to an electronic keypad lock. Greatest thing ever when there's multiple users! But I still keep a spare TT key in the van, in case the batteries die. Even if unlocked, it appears to make one last lock as the batteries die (found out the hard way).