On my Hymer there are two leisure batteries, one under the passenger seat and one under the drivers seat... plus one I had fitted under side bench, all three bought together with the same output and fitted by a Hymer dealer in Huddersfield..
It would be strange for there not to be a habitation battery, maybe a previous owner took it out to save buying a new one, which would make him "daft like brush!"..
Maybe but he has left no sign of cabling or holes or ANYTHING
Think i will just use that space under the passengers seat and add a battery or two