notsoswift, I don't claim to be an expert but it appears to me that there is something wrong. The battery charge is on pin 4 of the 7S socket with return on pin 3. [it is used for the permanent feed when the car engine is not running]. The fridge is powered from pin 6 with the return on pin 7. The supply for the fridge [pin 6] switches the relay within the van to charge the battery.
It appears to me that the van battery is actually running the fridge and there is no supply on pin 4 to charge it.
[I had a similar problem some years ago when the fuse in the supply to the fridge / charge relay in the car blew. The relay operated, as its activating voltage was still present from the car alternator. This had the effect of connecting the fridge to the van battery which rapidly discharged.
Please check ( without the van connected) that you have 12v on pin 4 with the -ve on pin 3 at all times, and that there is 12v on pin 6 with -ve on pin 7 when the engine is running.
Ray