darren , If pre 97 car / van the battery is charged via pin 2 (+) and pin 3 (-) on the 12 S. therefore with car engine running there should be 12 across pins 2 & 3 of the car socket.
On post 97 it is a bit more complicated. The permanent supply on pin 4 (+) with pin 3(-) is switched by a relay in the van to either power the van 12v system or charge the bayyery. The relay is switched by the fridge supply on pin 6(+) and pin 7 (-).
The car socket therefore should give 12v across pins 3 & 4 with the engine running or not and 12v across pins 6 & 7 with the engine running.
Note that if all that works out ok that the van / car switch on the dc distribution panel may need to be in the van position.