What do you wish...

There are those more learned than me on the subject, but it comes down to abuse! What I do know is bigger and heavier is better! More Lead, more acid, more electrons living longer! Forget  short term cranking power (there are purple blue pills for that), we're looking for long term abuse! I suspect you will hear more... 

fred

First reason is batteries in parallel can have issues charging and discharging equally, so putting the two 6 volt batteries in series gets around this problem.  Second is the deep cycle 6 volt batteries tend to be more robust than 12 volt deep cycle batteries - especially if you get 12 volt marine deep cycle batts, which aren't entirely deep cycle.  Another is there is a specific way to wire up 2 batteries in parallel and many people don't do it right, which amplifies problem #1, above.  The image below is the wrong way to do it, you must use the negative post on one battery, and the positive post of the other battery, to power the load.

I have an earlier Prodigy, but basically the same thing.  I think the setting is around 6.3.  I'm just thinking about the future, I've had my current controller for about 7 years so if it fails (or I get a new TV) I would probably get the RF.