One advantage to two 6V golf cart batteries in series, compared to two 12V
batteries in parallel, is in discharge cycle life. You can either discharge them deeper for the same number of discharges, or discharge them more times for the same depth of discharge.
Another advantage is that while they may not have significantly higher capacity (220AH vs 200AH) at the low discharge rates typically used when boondocking (i.e. 10A or less), they ARE significantly better at high discharge rates, such as the 25A rate used to establish "Reserve Capacity," or even higher, such as you'd find with heavy inverter usage. Keep in mind, though, that you have to compare their 25A rating to a 12.5A rate through each of the 12V
batteries in parallel. Here's the calculations of Peukert Exponent (n) and Peukert Capacity (Cp) for the Group 27 and Group 4C Lifelines. Note that there isn't much difference in 2 x the Cp of the 12 volt
batteries compared to the Cp of the 6 volt batteries, which indicates there will be little difference in their capacities at low current draws. But watch what that huge difference in exponent does as the current draw goes up. I've calculated the time for a 10A total draw (the 20 hour rate of the two 12V
batteries), a 25A total draw (the standard for establishing "Reserve Capacity", and a 100A and 200A total draws, powering a 1000W and 2000W inverter, for example:
(logT2 - logT1) / (logI1 - logI2) = n
(log186 - log1200) / log5 - log25) = n
n = 1.1583734592428565078682881631963
Cp = (5A^1.1583734592428565078682881631963) x 20 hours
Cp = 129.03225806451612903225806451613
Cp/I^n = Time so for 10A load, 10A/2 = 5A per battery:
= 20 hours
Cp/I^n = Time so for 25A load 25A/2 = 12.5A per battery:
= 6.9 hours (415 minutes)
Cp/I^n = Time so for 100A load, 100A/2 = 50A per battery:
= 1.4 hours (83 minutes)
Cp/I^n = Time so for 200A load, 200A/2 = 100A per battery:
= 0.62 hours (37 minutes)
(logT2 - logT1) / (logI1 - logI2) = n
(log492 - log1200) / (log11 - log25) = n
n = 1.0860161267351761034826747166678
Cp = (11^1.0860161267351761034826747166678) x 20 hours
Cp = 270.39536696992743815972924455649
Cp/I^n = Time so for 10A load:
= 22.2 hours (11% better)
Cp/I^n = Time, so for 25A load:
= 8.2 hours (492 minutes) (19% better)
Cp/I^n = Time so for 100A load:
= 1.8 hours (109 minutes) (31% better)
Cp/I^n = Time so for 200A load:
= 0.86 hours (51.4 minutes) (38% better)
Finally, there's a major difference in the failure modes of two 12V batteries in parallel vs two (only) 6V batteries in series.
If a cell opens in the 12V setup, you just continue to operate at 50% capacity. If a cell shorts in the 12V setup, the shorted battery discharges the good battery, but you can disconnect the bad battery, recharge the good one, and continue to operate at 50% capacity.
If a cell opens in the 6V setup, the whole system is dead. Period. If a cell shorts in the 6V setup, voltage drops to that of a dead 12V battery, even if all the other cells are fully charged. So essentially, the system is dead.
It may not make much difference boondocking in an RV, where you can hook up and head home, but if it happens a few days offshore and you don't have fuel to motor (and power your electronics) all the way home, it's more serious. That's why most boaters that go the 6V battery route have more than one pair of 6V batteries. The same is true for the big motorcoaches. There's been a bit of a trend away from the big 4D & 8D batteries in both, mainly because of the difficulty swapping them out, if required. It's about like swapping out a small diesel engine.
Finally, where space is concerned, there are differences. The 6 volt batteries have a smaller footprint, but are taller. Depending on your space available, one or the other setup may be better for that.