First: the "Advantages" of two six volt v/s one 12 volt.
There are two: WEIGHT and Cost, the six volt GC-2 Flooded wet cell is the single most produced battery in the US if not the World and thus it is lower in cost (per unit) to make and sell..
And being basically "half a 12 volt battery" (more on this in a second) it is way easier to "Wrangle" each half than to wrangle say a 4-D (The electrical equivalent).
Technically there is no difference between a pair of GC-2 in series and a 4D, Thus no advantage, Technically,, Just the logistic advantages.
in fact treat your six volts exactly as you would a 4D.
now AGM's Higher in cost. The biggest advantage of AGM is Maintence Free, and in some cases (IE: Motorcycle battery) no "This side up". The other advantages are basically meaningless in most RV installs (Faster recharge, well, less you upgrade your converter perhaps five mintues, Slower Self-discharge.. But you will be recharging before that makes a difference anyway, and a battery tender for winter storage is way cheaper than the difference between Flooded and AGM).
The AGMs I had, hooked to a Prosine 2.0 programmed for them (NOT to the house) lasted 5 years.. The Interstate U-2200's (On the house side for those five years and for the rest of their 9 year life running both the re-programmed Prosine and the house (Charged though by a Progressive Dynamics 9180 with wizard) lasted NINE years. And several times I took them down far below 50% and they recovered. had to jump start the generator once.