I don't think this is true. Not all bus drivers are Pluggers, or at least not all bus drivers that drive their buses home are Pluggers. I've only seen one house with a bus outside it, most are in a yard by a school. And the one house that did have a bus outside it was in a rural county.
Now, my uncle was a truck driver and his truck was often at his house. A trucker seems more Plugger-like. Maybe that's what Mr. Shepardson had in mind.