Are you sure you have not got a different fitting for a pump that fills it ?
On my Bailey i have a door on the side which, the pump that fills the onboard tank plugs in.
This pump differs from the usual one as it has two prongs on it...this pump might have got lost when you bought your van ? Check on the side for another fitting.
You need to drill a small hole a few inches away from the water filler to fit the 12v socket. If your van is similar to mine, the tank is under a seat at the rear, and if I remember correctly, there is also a 12v feed in there somewhere (it is a few years ago I fitted mine).
Drop the pump into the full water container (I use an AquaRoll), put the water pipe into the van's filler and plug in, and go get a glass of wine/beer. But don't leave the pump running for too long after it has emptied the AquaRoll (or whatever) - none of these pumps like running dry.