The R-pod had a 12V (nominal) system and uses deep-cycle batteries that can be deeply discharged without undue loss of life. 

 A 12V battery is six 2V (nominal) cells in series.  A 6V battery is 3 in series.  You can use two or more 6V batteries, or one, or more, 12V batteries each connected properly to arrive at the Amp-Hours capacity you need for your situation (think volume like gallons, liters or cubic miles).  There is additional bug dust to this that's important. 

It seems that two 6V GC2 batteries, in series, gives the largest A-Hr capacity that easily fits on the battery rack.  Should you find a 12V deep cycle battery of similar capacity, like a group 921 battery, it would work just as well as two 6V batteries.  Some would be lighter and cheaper, and the connections would be simpler. 

Plain old flooded batteries that you maintain by adding distilled water are usually the best choice, less expensive and better able to handle occasional overcharging. If you can't bring yourself to maintain the battery then get an AGM (absorbed glass mat) battery. Much more expensive though. 

Your decision will be based upon your camping 'lifestyle.'  Do you normally boondock or camp in areas where electricity is not available such as in national parks?  If that is the case the decision to go with a pair of 6v batteries is in your favor.  However if you usually camp where electricity is available and only occasionally camp where it is not, then the single 12v battery would be the obvious choice.

Then you have the weight factor on the tongue of your rpod verses what your tow vehicle can handle.  If you are towing with a full size pickup this is not an issue but if you are using a smaller SUV or lightweight pickup, then this becomes something also you will need to consider.  If you usually camp with electricity available and rarely boondock a single 12v battery is all you need.  The weight of a group size 12v deep cycle battery will weigh in at 50 - 55 lbs verses the mandatory 2 6 volt batteries you will have to purchase (to get to 12v) which will weigh in a 140 lbs or more for the pair.

When camping is your style simple (minimal use of electric) or do you have power hungry items that are needed when camping such as a CPAP machine or similar?

Finally, are you going to utilize solar or a generator?  If so and you don't boondock all that often I would go lightweight and opt for the single 12 volt option.

I usually boondock but only stay put in one place for 6 days at most.  Even in cold weather (night temps 38 - 50 degrees) I will get 3 days each out of my 10 year old 12v group size 24 deep cycle Interstate batteries.  After 3 days they are down to 60% S.O.C. or 12.2v and then switched over to the other battery.   IF either solar or generator were utilized (which i don't) then staying in one place would be much longer. 

Also remember that as you travel your tow vehicle will be charging the battery on your R-Pod so when you arrive at your destination the pods battery should be fully charged.  This is taking into account that as you travel you will be running the fridge on propane, that travel time will be more than just a couple hours and that you haven't discharged your battery more than 50% S.O.C.

Sailor, yes both batteries are mounted on the a-frame as is also a pair of 20# propane tanks.

Those monitors are notorious for being inaccurate.

The best way to test the battery is use a meter that shows the amperage(in volts) and then you can also convert that to a state of charge(SOC) percentage.

The freshwater tank usually can be visually checked by simply looking under the pod and viewing the level of water in the translucent tank.

You can open the flush valve on the toilet and look directly down into the black tank or even use a disposable stick to manually measure.

the gray tank is always a guess.

The thing to remember is that you have 30 gallons in the fresh tank and both the gray and black tank hold 30 gallons each-so it is very unlikely that you will overfill either black or gray.

there are after market sensors that people buy and use. I have no experience with them

Safe travels

Vann