Have a couple 12's from Costco and never had a problem. Saved a few $ over other options too. Have been meaning to get a solar panel set up but never got around to it after buying my Yamaha generator.
A decent group 24 12v deep cycle battery will have approx. 90 amp hours. If you hook two in parallel you still get 12v but double your amp hours to 180.
A decent 6v deep cycle battery will have approx. 230 amp hours. If you hook two in series you get twelve volts but the same 230 amp hours.
Everything you use can be calculated into amp hours. Two six volts give you more amp hours= longer battery life. You want to try and never take your batteries below 50% SOC and they will last longer.