Sorry, but a warrant is required to search my vehicle, no matter where it’s parked.
Warrant or no warrant, a private property owner is free to prohibit anything from his property, guns included. A private property owner is also free to require a search of every vehicle that enters his property. Of course, you would be free to say no, but the property owner could then prohibit you from entering the property.
I would never work for a company that did this, of course, and I assume most here would not either, but that’s the whole point of a free market - it’s our decision, not the government’s.