If someone wants to get in they will, so not leaving things out in plain site and temping the bad guys is the best thing to do. I agree with those that leave valuables in the camper as opposed to the truck cab, it seems to makes sense that the double lock/deadbolt combination and not being able to really see what's inside of the camper might reduce temptation for some. I really agree with not having any bumper stickers that might incite unwanted attention to your rig.
smoke