You may be right about 2nd amendment rights, but what is your defense for the violation of the 4th amendment? I believe that the Constitution trumps private property rights in this case.
Fourth Amendment - Search and Seizure
"The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized."
If you accept a job, with the understanding that your vehicle may be searched for guns, then the search is not unreasonable.
Issuing a blanket statement forbidding guns on a property is neither search nor seizure. Again, the individual is not required to enter the property. It's his choice, but if he chooses to do so then he has to abide by the owner's requirements.
By your logic you should be allowed to sue a business anytime you walk through a metal detector. The Oak. law will be struck down since it is laughable.