I think one of the biggest deterents, assuming others are relatively nearby and you are not boondocking alone, is NOISE.
If your tow vehicle has a keyless entry/alarm with panic button you could set it off remotely from inside the TM if someone was trying to get in.
They also sell very LOUD (130db) panic alarms you could keep in the TM.
Most people looking for trouble aren't looking to get caught and would likely flee with that kind of noise/attention being drawn to them.
Found this site (UK, but they have US re-sellers):
http://www.ilasecurity.com/uk/
--jim