We are new to the A-Frame world and I too had these same questions, here is what I learned.
There has been a bunch of consolidation in the battery industry. I have had Optima batteries in the past and thought highly of them. Sadly Optima was bought by Johnson Controls, which move their production to Mexico and the quality suffered and has never improved. A local vendor said he receives more warranty issues with Optima than any other battery.
AGM batteries have higher CCA compared to a wet cell or flooded battery, but that's not what you are looking for in a camper battery. Amp Hours is what you want, and it's just not possible to get the amp hrs out of a AGM battery that you can out of 2 6V flooded batteries. The cost per amp hr makes the flooded or traditional batteries the best choice in my opinion.
I think what less understood is how you should charge your batteries...I really didn't know much beyond plugging them into a trickle charger. I found an article that does a great job explaining solar and charging. I found it very helpful and as a result I'm comfortable with my solar charging system maintaining my battery all year round, even while in storage for the winter.
I think it's worth a read!