As much as I support "law and order" I have to agree with the manager. It is private property, and if they don't have a warrant, they can not go fishing in the parking lot.
Now, if they were looking for a specific truck, then it would be different.
In California they have mandatory truck weight stations on all Freeways. I assume that is where they do a cross check of licence and vin numbers.
New Mexico has weigh stations on the freeways also.