Unfortunately traps that cause injury or death are not legal unless you are directly protecting lives by arming them. Live traps are legal, but impractical when trying to protect a vehicle. The best you can do is reduce temptation to steal items by keeping them out of view and adding enough deterrence that it will take too long to bypass or cause too much attention.
Self powered high db noise makers and strobes that activate on a trigger signal and do not timeout until the power source is depleted can be placed in locations that are extremely inaccessible.
Vehicle ignition cutoffs, fuel starvation and brake line locks are some methods to keep your vehicle disabled beyond mechanical/physical means.
Something that was not discussed to deter theft is to leave your slides open and jacks grounded when you park your camper. Now the thief will have to break into the truck to get it started and into the camper to close things up before he can attempt to leave without creating undue attention. He will also have to ask himself whether you are actually gone or just sleeping in the back...
