Gotta love magnetic signs. Once the freight is delivered, the signs come down and get stowed, then the vehicle is as off-duty as I am until the next run.
I would say a lot depends on who actually OWNS the vehicle. If it's your vehicle that you're driving, the company would have to have a lot of crust to tell you that you can't drive your van while you're off-duty. If it's a company-owned van and they're paying you to drive their van, then that's a bit different. They DO have rights to say how THEIR vehicles are used by the people who drive for them. So-- who bought and paid for the vehicle you're driving?