HuntingDog I feel your pain and frustration. I agree a hidden key is not the best solution but it is somewhat a foolproof way to avoid the situation.
Another option is to simply not lock the door with the key when there is someone inside. The key lock does not provide anymore security and is simply more trouble that it's worth in your scenario.
If someone is aiming to break in and manage to defeat the deadbolt the other latch will not hinder, or slow them down at all.
Forget the locks. RV doors and frames are simply not the substantial. The door and frame are not sturdy enough to stop a determined intruder with or without the additional key lock.
There is no point in designing a heavy duty lock if its going to be attached to a paper bag!
If you want to provide additional security a few slide bolts locked by the person on the inside may help.