hope this helps!
pop to a local motor spares or caravan spares shop.
Asl for a RING CARAVAN BATTERY/SPLIT charging relay. Most good shops will stock them,
You dont need the cable from the alternator with this type.
Just pos+ and neg- from your van battery.
When your leisure battery runs low it will not start draining your van battery, but when your van is running your leisure battery will charge up.