auxiliary batteries

Hello I want to enquire if anyone could confirm for me if selecting the second battery option on extras list when ordering a new van, is this in fact a leisure battery with a split charger that can be used when ignition is turned off to be used for running battery operated items when camped etc...

I'm looking to run a cable from the main battery into the cabin. Anyone know if there is an easy route through, maybe using an existing path, or do I have to think about drilling a new hole in the engine bay bulkhead (gulp). Any advice much appreciated.

While I save alittle more for the install, I am looking to source advise on pros/cons and links to house the 110ah battery under vehicle. I do not have room under the front seats and due to changable internal use, no room to the rear. Thanks. @travelvolts

there's a thick cable under the passenger seat that I'm wondering what it is. It has a barcode on it? so it looks original. I'm fitting a leisure battery /ctek solar charger - so am hoping it could be useful. Guy