Usually, but could vary on earlier vans. when attached to the car and the engine is [usually] running, the 12v from the car will run internal lights and pump for the sink, along with fridge and charge battery if your car is wired for this. When on site the 12v should run the internal lights and the sink. It will not power the fridge as it would drain the battery after about 1/2 a day. Also your exterior/awning light should not work when the van is attached to the car.
Colin