By now, this is a chorus, but I'll chime in.
In the world of flooded cell (lead acid) batteries...these observations change when applied to other battery technologies.
Why two 6 volt in series rather than 2 12 volts in parallel.
The advantage of 6 volt batteries.
~ Slightly more robust build.
~ True deep cycle...not "marine" or mixed use.
~ Slightly more usable amp hours in the battery bank.
A good, reasonably priced 6 volt "golf cart" battery will weigh in around 64 to 68 pounds. A competitive 12 volt true deep cycle may weigh in around 60 pounds. As with 1960's vintage Cadillacs, that "ground hugging" weight matters. Thicker plates and "more lead" make for stouter batteries.
Here's a fair Apples-to-Apples comparison:
12 Volt true deep cycle Group 31:
https://www.batteriesplus.com/productdetails/sli31mdc
6 Volt true deep cycle type GC2:
https://www.batteriesplus.com/productdetails/sligc115
Exactly the same price.
This pair of 12 volts offers 200 amp hours - you get to use 50% (rule of thumb) for 100 total usable AH.
This pair of 6 volts offers 230 total AH (it does not double, because in series the voltage doubles) for a net usable AH of 115.
15 more AH doesn't sound like much until you realize that a standard group 24 12 volt battery that comes on the tongue of most new RVs can only deliver a grand total of about 35 USABLE AH. 15 AH is damned near half of a group 24...extra for the same price: $130/battery. Battleborn LiFePo's by the way run close to a Kilobuck a piece.
So, when the sparks hit the wires, you have enough extra "in the tank" to run your furnace overnight in temperate weather.
Downsides of 6 volt batteries? If one dies, you're toast. With 2 x 12's you can always run on one if the other dies. With 2 x 6's, you can't run on 6 volts.
Now the "gold standard" for inexpensive battery banks is 4 x 6 volt GC2's. Best of both worlds. If any one fails, you still have a pair that can make 12 volts. And, as battery banks go, the make available to you 230 AH of usable power, which is quite a bit if you don't do something silly like suck them dry with a big inverter or have an RV with a "residential" (compressor) fridge.
If you're one to be tethered by a power cord to RV parks, no big deal. But if you boondock, a residential fridge is a real joy killer. If you boondock, you need a 2 way fridge that will run on propane.
A simple, brutal, fact of life. 2 x 5 gallon tanks of propane (actually 4.5 gallons each) contain MORE THAN 20,000 AMP HOURS OF ENERGY. Damned near 100 times (about 87 times) the energy of the gold standard 4 x 6 volt golf cart battery bank!! BATTERIES SUCK as energy storage devices.

With that in mind, those 15 extra amp hours in the pair of 6 volts look mighty good.