I've got a Combo. It's quite easy to run a (fused) cable from the battery through the bulkhead on the nearside, tucked up under the plastic panel below the door.
There is a wire that comes out of the alternator to the instrument panel alternator warning light, (blue I think) , cut it, rejoin it with a cable connector, add an extra wire, so you have one wire in one side of the connector and two in the other side, then you run it through the same route as the cable from the battery to the relay.
The cable from the Van battery is connected to the leisure battery and the relay behind the driver's seat or wherever you have your battery.
If you're unsure an auto electrician would sort it quickly for you.