We don’t allow people to sleep in the back of their cars or vans unless they are modified and certified camper vans. This is all to do with isolating the battery. A campervan can switch between running the engine and the battery. In a van if the engine is run to use the battery to do something simple like charging a phone, there is no ventilation in the back of a van. It can be quite dangerous and also does present some child protection issues.