Hi BFL13,
Your eyes may be open--but they are blinded by not seeing Puekert. If you use the twin twelves with the twin sixes as one large bank feeding the inverter you will have more power longer with a lower over all depth of discharge.
Many inverters alarm at 11.5 volts.
I would not take six volt batteries to 30% state of charge let alone 20%.
The twin twelve volts in my former RV did last for 9.5 years on usage of 50 days per year, until the new owner thought that the old magnatronic converter should be plugged in for months at a time. Even after that, the solar system "reserected" the bank enough to allow for lighting and running the fridge.
