Hi
The relay should be connected to a charge light wire as the trigger, this is different to being ignition controled an ignition controled unit would connect the front and rear battereries merely by switching the ignition on and not having the engine running and producing power.
The Charge light wire only becomes positive when the alternator is producing power.
Fuse, if the battery for ighting is ever taken really low discharge upon start up it will try to draw full ampage for a short while, which can blow the fuse, I would try using a fuse a couple of amps down on the cigarette lighters fuse and see howyou get on.
Wire thickness gauged for 30- 40 Amps
How are you going to vent the leisure battery? as it got a vent tube? Make sure the battery is really well secured otherwise an emergency stop could prove less than entertaining. Also with it being in an accessed area do make sure the terminals are well insulated against accidental contact.
PS all are welcomed here, whethor you have no van or £250,000 worth