Rig the door so it only unlocks from the OUTSIDE with an electric lock you can control with your truck's key fob lock/unlock feature.
Now when someone goes into the camper when the truck's doors are locked, the door to the camper will lock BEHIND the thief when he enters the camper and he won't be able to get out. All you need do is drive up to the woods and take care of the vermin...
Also, you might wire the door knob with a battery operated electric fence charger. When he grabs the door handle to go in, ZAP!!