Type of cables to use for connecting 6v batteries?

Thanks for that link.

OP: You're running 6v batteries in series. Your battery connections need to at least match your output, so 6ga is right. 4ga is fine.

If you use that inverter, you'll have to go to 2ga, so something to consider for now.

I, like others, use a real battery disconnect unless you've got the holy-grail of RVs with no parasitic draw.