what gauge wire between two batteries?

Good lord id hope that the charger is stronger than 2 to 6 amps. Are you talking about engine charging or shore power? If shore charging it's usually more like 50 amps (see that gizmo I linked to in a precious response). If engine, some systems use a cheesy relay system that simply connects the two battery banks when the engine is running (or more accurately when the key is turned). Mine has a dual alternator that charges each at around 50 amps.

I use and love the trimetric battery monitor, which tells me the exact charging rate:

http://www.solar-electric.com/trtmbamosy1.html?gclid=CMKA_Ou2r80CFVUkgQodQtwE9Q