AHs are AHs. 2 6V batterys totalling 220AHs are the same as a single 12V battery, probably a 4D case size, rated at the same AHs. The difference is the 4D battery weighs about the same as the 2 6Vs combined & the single 4D eliminates 2 connections joining the 6Vs where there is potential for failure or leakage.
I am a big fan of AGMs, having already invested in 3 stage programable pure sine wave inverter charger. For us the dry camping is more than the occasional overnight at Walmart. We like to be comfortable doing at least 3 or 4 nights offgrid doing things like music festivals & the Albquerque Balloon Fiesta for a week. For those reasons the expense of the AGM is well worth it.
For the FHU camper with only the occasional need for drycamping overnight the wet cell battery bank, single 4D or 2 6Vs are just fine.
