Perimeter beams tied to an alarm, a loud alarm, is one approach. Also, you mention having cameras but, evidently, not very good ones. This is not an item on which to economize, as you are finding out.
Good equipment costs more for a reason. Get better cameras so that when you show the police your surveillance tapes, they can better make out who the culprits are. Given the location of your facility, law enforcement may already have its suspicions; your video gives them something concrete the pursue.
Some exterior lighting wouldn't hurt, either.