The whole battery discussion swings on what the goals are.
If you want the minimum number of cells then six individual two volt cells is the best of all worlds (600 amp-hours per cell, and up).
If there is room for an odd number of batteries and the choice is between six volt and twelve volt jars, twelve volt wins on capacity.
If there is room for only two batteries twelve volt wins on redundancy and six volt wins on the number of cells.
If a large inverter (1500 watts and up) is to be run at "full bore" then twelve volt wins even against six six volt (assuming equal amp-hours of capacity), due to the larger voltage drop exhibited by the thicker plates.
Flooded cells remain, for the time being, the "best bang for the buck", but the trade off is regular maintenance and a pair of wool pants.
If one does go to agm chemistry, it would be wise to heed the advice of the particular maker on charging parameters, given that the cost of such batteries may far exceed the cost of a good converter.
Most RV's are woefully low on amp-hours of capacity, and many folks abuse their battery bank by going to far lower states of charge than is good for longevity. To me, it is a no brainer to maximize the capacity.
Balanced wiring for battery banks is often hit and miss (mostly miss, I fear). It should be addressed.
No matter what voltage or chemistry is used, good charging practice need to be followed. If that is done, 2 volt, 12 volt, and 6 volt battery banks may last many years.
So, find out the goals, use what ever works for a battery bank, and go camping.
For my further thoughts on battery banks surf here: