I have a horizon and read the manual some time back, both batteries are housed under front seats, one in each, i cannnot be sure but I think the leisure battery is under the drivers (RHD) seat. But I also remember reading that they are linked however the rear power outlets and lighting is connected to the leisure battery. Not sure about charging but I would guess that connecting a optimate type socket to the terminals would allow you to plug whatever you want in.
You could possibly use one of the 12v outlets?
Details are in the manual supplement I think.