Montana high country 338DB needs more batteries

Two 6V batteries can have higher capacity AH then one 12V IF the 6Vs have higher capacity AH to begin with

Two 6v Batteries wired in 'series' will double the voltage but have same capacity AH



To increase capacity AH requires 'paralleling' of batteries


As for charging......check which Brand/Model of converter you have.
A good 3 stage should be compatible with AGMs

But two 12V flooded are less expensive and can double your capacity AH