This would be my plan for running two 6VDC batteries in series using the Trojan T105 batteries. Your COSTCO 6VDC batteries are similar but have slightly less performance.

Ignore the 1500WATT AIMs INVERTER hookup that was for my four 12VDC battery hookup...
Check out BLUE SEA for high quality marine grade high current battery switches...
I ordered my ANCOR battery cable in bulk and used ANCOR high quality ring terminals. Then used this hydraulic hand crimper to make up all my own cables...



Or just find some old welding cables lengths, cheap ring terminal, black tape, hacksaw, and a hammer... That works too...
Roy Ken