I got a few suggestions:

A. You can get a Geo-fencing GPS which will track the truck as well as limit the area to which it can go. If they force it to go past the fence, you can set the device so it alerts you or kills the engine. This will also send you a SMS indicating its location.

B. Install a few DVRs inside - one facing outwards so you can see if your driver is careful, one facing the cabin and one inside the storage area so you can see if they're opening it and taking out items.

C. Bring all staff who steal to the police no matter how small the item is.

D. This is the most difficult but try to raise their morale by instilling pride in them for their work. Do you have internal incentive programs?

I believe you can only charge them for the amount that they stole and not more than that. They are actually still innocent until proven guilty so the burden of proof is on you. They can get back to you with false accusation and defamation.

Good luck man!