I bought a cheapy wireless motion detector. I bought originally to make sure my little one didn't sneak out the front door in the middle of the night but it works good for scare em alarm. Has 2 settings; motion detector that chirps like walking in a dept store and alarm that sets off a siren. I used chirp for the little one and alarm for security. It has a wireless remote that you can arm it from outside the camper and if anything moves in the living area it sets off a pretty loud alarm. Given, you can walk over and turn the power switch off on it but my guess is it would scare someone off pretty quickly.
This isn't the one I have but its similar. Really inexpensive.
http://www.amazon.com/Wireless-Motio.../dp/B002B96XWW
As far as securing the camper. I have a pad lock I put on my pin box tripod but bolt cutters would snap right through it. I think your more dependent on the campground and neighbors if you're leaving unattended for weeks.