Ron,
As you know the important thing is to make sure that your RV winds up with true deep cycle batteries, whether 6V or 12V.
For 12V batteries, true deep cycle versions are difficult to find and probably the easiest way to get them are in AGM versions.
Lifeline AGM batteries get a lot of attention here in the forums, but I'm not sure they're "the best" AGM game in town. I wouldn't be afraid of Rolls Surrette AGM batteries with their sterling reputation around the world.
If you have the room, you might consider a couple of these 145 AH true deep cycle Rolls Surrette 12V AGM batteries (for a total amp hours of 290). I you like lead, notice that they weight over 97 lbs. of each (download the .pdf sheet to see all specs). They seem to have about the usual automotive size base dimensions - but a bit taller. Note that their recommended float voltage is 13.5V to 13.8V, so standard converters are exactly right for connecting to them with no long term damage. Two of these get you 290 AH of true deep cycle 12V power (I assume that the current ultimate RV battery - lithium - is not on your horizon!):
http://www.dcbattery.com/rollssurretteagm_s12-160agm.html
