Are you testing at the pin with the engine running ?
There is, should only be, a supply to the fridge pin and the caravan battery charge pin, only when the car engine is running and the alt has reached a specific voltage.
This is to ensure that with the engine stationary supplies to both are cut off to prevent the car battery being discharged.
However there is a permanent supply to the van, engine off, this goes to the habitation relay in the van, or on older ones, the van-off-car switch, this is to allow the 12v lights, water pump and toilet pump to be used on comfort stops, but has no connection to the van battery.
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Common sense isn't a gift, it's a punishment because you have to deal with everyone who doesn't have it. 
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