If you currently have one 12V battery, and you have room for two side-by-side, consider two-6V golf cart batteries. They are a couple inches taller so keep that in-mind. Two true deep cycle 6V golf cart batteries will charge and discharge very evenly for much better performance than two 12V batteries.
Just about every on-board built-in RV battery charger can charge two batteries. A pair of 12V hooked up in parallel, or a pair of 6V batteries hooked up in series, it makes no difference to the charger.
A number of motor home manufactures have been installing a pair of 6V golf cart batteries for greatly improved battery performance and reserve power, so it's not a crazy idea.....and there is very little difference in the purchase price. We bought our 6V golf cart batteries from Sam's Club. We bought 6V AGM versions in 2015 and they are still performing wonderfully.
I would rather have two 6V conventional wet acid batteries than three 12V batteries. The 12Vs discharge and charge unevenly making for inefficiency and battery boil-overs.