I was kinda hoping that someone with an 05 configured like mine would chime in with an exact location for the 3rd battery that involved no extra work and also miraculously makes the boat run flat and level all the time without ever touching a tab. :smt001
Let me know if you figure it out - I'd be interested in that, too!
The "Supplement Manual" that came with my boat had bilge layouts for all possible combinations. Maybe yours does, too? I don't know - never looked at a manual for an '05+.