[bushpilot]

I have a blue sea (like the one in the link) - on my f350, linking in a 3rd battery (mounted under the truck on the frame rail (for the stereo system).

the blue sea has an over ride (dash switch) that allows you to join all batteries to the (starting) circuit...if the dash switch is too easy for ya...you can MANUALLY over ride the relay by turning a switch ON the relay itself...

over rides only last for 1 minute or so (to prevent killing ALL the batteries).

nice thing about the blue sea - if CHARGE output is below 13.4v then it WILL NOT link in the 3rd (or house) batteries.....if the 3rd (house) battery is below 12v, it will also NOT join the battery to the mains.

pretty cool set up, not cheap, but its smart enough to prevent a bad alternator or bad battery from pulling the rest down with it and killing EVERY battery in the configuration.