I had my truck broken into in a parking lot, is the property liable?

It is unlikely but depends on you developing facts that the establishment knew or should have known of many prior break ins and did not do anything to warn you of the high incidence of criminal activity in their parking lot and that they failed to take proper steps to secure the parking lot.
Your case would be a negligent security type case and these are typically very fact intensive and expensive to develop necessary proof and bring the case. Unless your loss is great the cost of this type of case would likely exceed your loss. Failing your ability to prove the case, the establishment would not be liable. You may still contact their insurance carrier and demand they pay and perhaps they will.