Does anyone have any familiarity with the TCW integrated backup battery system sold by Vans? It's pricey but it sounds like it might be more practical than installing an additional PC-680 to backup your critical electrics. The thing is I've never heard of it and I don't know anyone else who has. It does kind of sound like it was designed specifically for the job.
http://www.tcwtech.com/IBBS.htm
http://www.tcwtech.com/IBBS.htm