I have a factory dual battery set-up, what now?

My truck (1998 GMC Sierra 2500 4x4 7.4L) has a place for a second battery. I would like to get a second battery connected, mostly to run my camper, but also for back-up. I have a hunch the isolation etc is built in. Where can I get info on how to hook up and use a second battery? The owners manual was no help, neither was Haynes.