Answer:
It varies by state. In New York State, there was a debate ongoing about whether a company vehicle would be included as a place of work. They ultimately came to the conclusion for sleeper berth equipped tractors that they were not, since there was a living space within the vehicle.
Your best bet is to look at what the laws are for your state in regards to this. Some states may look at them no different than any other vehicle, whereas others may consider them a workplace.
Another factor to consider is what the policy is of the company which owns the vehicle, as well. They may prohibit it, even if the state law does not.