Donn - the problem that I ran into with two 12 volts is the size of the 12 volt vs 6 volt bank and the additional cost to the 12-volts.
Trojan T-145 6 volt in series offer 260Ah. They are 10 3/8" long, 7 1/8" wide, and 11 5/8" high. Total weight is 144lbs.
Trojan T-145 12-volt in parallel offer 280Ah. They are 12 7/8" long, 7 1/8" wide, and 11 11/16" high. Total weight is 156lbs.
My tray would never allow that much length and I didn't feel like modifying it. Granted this might not be an issue for the OP.
The 12-volt bank would have cost $150 more. Hard to justify.
DW, 3 Kids and our Goldens
2012 Shamrock 233S
2008 Toyota Sequoia 5.7L 4WD