I bought one of these to put inside my trailer while at the storage lot.
Amazon.com: Mini Alarm Motion Detector and Remote Control with 10,000 possible codes: Home Improvement
It is cheap, and give me a slight peace of mind. I figure that if some punk is going to break in for the TV's, this might scare them off. It works pretty good and runs on batteries for about a month without changing.
I also have a hitch lock.
I figure these things will keep a honest person honest, but if someone really wanted something, that they could get it with what ever I have installed.