I would look into a car alarm system.
There are lots of options out there.
They usually have a main unit that you can accept inputs from:
Remote for (activation/deactivation of alarm)
High Output multi-tone Sirens
Dual Stage Shock Sensors
Some can even notify you when the alarm has been tripped.
So if you stop at a restaurant, grocery store, or sight-seeing attraction
and someone backs into your trailer you will know.
Best thing they will run off your 12 volt
Good luck, and hope you never really need it.