You don't have to choose Duracell (or Lifeline) AGM batteries.
Here's two other models of deep cycle batteries to choose from. Both of these offer plenty of amp-hours per battery and are to be floated at around 13.6 volts. Both also charge up fairly well (because AGM batteries have low internal resistance) on only 13.6 volts if you would like to stay with a stock RV converter for maintaining them.
We used two of these Interstates in our RV for around 8 years with no temperature compensation and on only the stock 13.6V converter plus engine alternator:
http://www.interstatebatteries.com/p/werker-wka12-100c-fr-dcm0100?dsNav=N~2147384796-2147384903We now have two of these Fullrivers in our RV with no temperature compensation and on only the stock 13.6V converter plus engine alternator:
http://www.fullriver.com/products/admin/upfile/DC115-12A.pdf