I had the same problem recently which turned out to be the 5 pin relay. While not knowing your specific 'van (ours is over 20 years old) I have been led to believe that things in this department have remained fairly static, so with this in mind-------------------- Under the bonnet, probably next to the battery, there should be a small block with two one inch cubes in it, these are both the same (30 amp 5 pin relays), one controls the split charging and 'tother the fridge. Being a total numpty, I had no idea which was which, so I replaced them both, total cost £7.00, the fridge now runs on 12v when the engine is running.
There are many wise and wonderful folks here, so if I am wrong, they will offer a better idea. But for what its worth, this is my experience