This is a 12V system and 6', not 6V or whatever chart you are using to come up with the wrong answer.
4 AWG (not 4/0) is good for 200 amps and 20'... more than enough for the OP's system in a small Class A with probably a 1000W inverter. Even if he has a 2000W (167 amps at 12V) inverter a 4 gauge wire is still large enough.
OP, the short jumpers between the two 6V batteries and the negative ones to the frame can be 4 AWG as there is no good reason to run two 6' wires. If you have a 2000W inverter then to make some on here happy... 2 AWG for the 6' run otherwise 4 AWG. If you were using four 12V batteries then two 6' wires so you could balance the system would be worth it.
