It varies from van to van. You need to consult your handbook.
On our 1998 Sterling Emerald it was always "van". "car" was only in an emergency, say if you needed to power the electrics from the car battery if the van battery was flat. And this only when stopped. Centre position was "off".
Vans younger than 1997 generally have a habitation relay that cuts all power to the van when towing, so "off" is only really required when the van is not in use.
12v power to the fridge is always separate, you cannot choose where it comes from.
You can download a handbook from Swift if you do not have one.