Ron,
I know that it's too late for you now, but ...... one thing I watched out for when I bought the (12V) AGM batteries for our RV was how well the charger's float voltage matched the recommended float voltage of whatever AGM batteries I was considering.
Not all AGM batteries have the same recommened float voltage. For example with respect to 12V AGM batteries - some manufacturers recommend around 13.2 volts for floating of their AGMs (the same as most wet cell batteries), and some manufacturers recommend 13.5 volts to 13.8 volts for floating of their AGMs.
For your new 6 volt AGM batteries - add together their recommended float voltage to see if the sum about equals the float voltage output of whatever ("12V") charger you're going to use with them.
My main concern was I didn't want my AGM batteries to week after week sit at a too-high or too-low float voltage. Too-high may very gradually over time maintain them at a float voltage that evaporates away their irreplaceable water, and too-low may very gradually over time maintain them at a float voltage too low to prevent the AGM equivalent of sulphation. These two scenarios can of course be exacerbated, depending upon the air temperature(s) the batteries are exposed to when sitting in their compartment day after day - whether the RV is camping and on float, or stored inside a building or stored outside and on float.
