Our 2011 Landmark came with a single Group 27 provided by the dealer. Our friend bought a similar 2012 Landmark that came with a Group 24 provided by the dealer. I'd be surprised if your dealer provided a battery larger than Group 24 for your Prowler.
I think there are varying opinions on two 6V in series versus two 12V in parallel. Perhaps others with 6V experience will chime in.