Dave,
I bought a personnel alarm for under a tenner and mounted it on the unit inside the door (double sided tape), when leaving the van i hook the actuating pull cord over the door latch, if any one enters the alarm is activated, dont know the decibels but its deafening, and a cheap alternative, I asked my insurance co if this was ok, and was told yes it is an intruder alarm,not hi tech but it works, and is right in your face when it goes off.
TONY A.