The DC system of an RV is 12V, so you do need a 12V battery. With a lead-acid battery that means six cells in series, as each cell is (nominally) 2V.
With golf cart batteries, the most common ones are 6V batteries (i.e. three cells). To get 12V, you need to install them in pairs, each pair wired in series, forming a single 12V battery that happens to be in two physical sections. You can also use one or more 12V batteries (six cells in one case) wired together in parallel, whether these are golf cart batteries or other 12V batteries. The performance and capacity per dollar usually favors the 6V golf cart batteries unless you have pretty limited 12V power needs served by a single 12V battery. I suspect with a little fridge running off an inverter you'd need more than the single battery.
