Trailer alarms are a great idea: so long as you close the trailer every time you swap some piece of equipment. I have actually heard of a company which would leave one man with the truck/trailer on one property to prevent theft while the rest of the crew worked the site.
Moving on; Have a car alarm on the trailer with door magnets, shock sensor and proximity alarm. The shock sensor was to set the thing of when someone was messing with the locks and before the doors were open. the proximity sensor was to be able to leave the door open yet set the alarm of when someone entered the trailer. Problems occurred in severe rainstorms, and with falling acorns or branches which would set the alarm of (usually at 2 in the morning of course), strong winds would also trigger the alarm by flexing he roof which would create a "shock". The proximity sensor was an interesting experiment, but could trigger the alarm when a truck passed by. I usually kept it off!