Wouldn't a split charge system be easier to fit & use?
I had one battery used for running & starting my old Disco and the second one for running the winch, spot lights (including the roof mounted ones), rear power socket and inverter.
The winch was run direct off the battery and I installed a separate fuse box for everything else.
Both batteries were deep cycle batteries of the same size & capacity so I could swap them around if needed.
I found this photo of it:
![Click to enlarge](http://i41.photobucket.com/albums/e267/heb-photos/bat03.jpg)
The fuse box & two light relays are fitted to the inner wing, behind the air filter. The split charge relay was on the other side of the engine bay.