Hi Trout, welcome to your 24/7 owners manual. A lot of what to replace your 6 volt batteries will depend on your camping 'style.' Do you normally camp where electric is available? If so a single 12 v deep cycle marine battery will be all you need. If you normally boondock than you have a couple options that either being a pair of 6 volt batteries wired in series (to give 12 volts) or a pair of 12 v that could be used individually (which is what I do) or hooked up together in parallel to give 12 volts but increase your amp hours. A pair of 12 v will weigh at least 25 lbs less than a pair of 6 v batteries, something to consider for your a-frame.
My two 12v Interstate batteries (group size 24, deep cycle marine) are now 10 years old. We usually boondock and they have and continue to perform well. With proper maintenance they will last . . . . . . I am not sure? We do expect to get a minimum of 6 days total with our batteries when off grid - this is when night temps are in the 40's and furnace gets used a lot. In warmer weather they obviously will go much longer than 6 days. Am amazed how well these have performed. Important for battery life is not to draw down below 60% S.O.C. or 12.2 v (on 12 volt battery) before recharging.
Your onboard battery converter in your Rpod will work and charge your two 6 volt batteries together and/or 12v batteries, no need to change out.
Batteries should last more than four years. Again, if you are not using your Rpod for a week or longer either disconnect the ground (-) cable or install a battery disconnect so your battery doesn't get drained down to far when sitting idle for long periods of time.