Avan Cruiseliner technical advice please

Hi. As the new owners of a 2007 Avan Cruiseliner, we took it away last week to work out all its bits and pieces. We have one problem I'm hoping someone with a Cruiseliner can help me with. The exterior door at the rear that gives access to the jack from the outside is locked and won't unlock with any of the keys the dealer gave us. They say they don't have any more keys and that we should have a different one for every lock on the van as they are all keyed individually. We can open the water inlet and the front boot with the same key, but nothing seems to open the jack access door. Can someone please tell me if it has a separate key or we perhaps just have a very sticky lock on that door?

Thanks
Sandra