Prior to reading this article, I thought a "Truck Gun" was primarily a lower-cost handgun that was kept in a vehicle for emergencies, and was generally one that you didn't mind getting scratched/scuffed up. Most of the people I know consider things like Hi-Points and Rock Island or Taurus revolvers, or older guns that they don't shoot often, to be truck guns.
I know a few people who prefer firearms like the Mossberg Shockwave/Remington Tac-14, or maybe a 10-22 Takedown, but I'm not sure that anyone I know uses a traditional long gun for this purpose. I spend most of my time in the city, so I would not be at all surprised if long guns normally filled this role in more rural areas.