Although it's an option, most vans have a set of three 'customer' terminals on the back of the driver's seat base, each fused at 60A. It's here you should make your leisure battery connection(s).
If you're fitting a single passenger seat on a driver's seat base, you can buy a 110Ah leisure battery the same size as the van battery, like
this one, or you could always put two batteries in there. That's the plan for mine.
If I do fit two batteries, I'm undecided yet as to whether to wire them in parallel or to split the leisure circuits up over two batteries and effectively have two independent leisure systems.
The standard van alternator at 150A should be sufficient to charge all systems but you can buy a 175A version. A search for Transit ice cream van alternator should point you towards one.
Drink moderately, eat healthily, exercise regularly, die anyway.