Its one thing to sleep while on the clock, but i dont see a problem with taking a nap in your car for your lunch break.
I have done it before - not in a van though. I used to drive an eclipse and park in a large underground parking garage. After an all- nighter it was heaven to go down there and take a 1 hour power nap. I would just turn the car on, lay the seat down, and set an alarm.