First; 6v vs12v, there is no definitive data that says one is better than the other, only opinion. Thats fine, but to make the statement one is better over the other is not accurate.
Batteries are difficult to rate in that the stated specs may not live up to their real life performance. There is no reliabe independent ratings on deep cycle batteries in real world RV usage that I have ever encountered. If someone knows of independent ratings, please by all means chime in.
That being said, if you plan to be off the grid while in camping mode, buy the highest spec(amp hr, etc) deep cycle battery(s) that will fit in your storage compartment. Learn all about battery does and don'ts by researching here and other excellent sources on the net.
You will find, like I have, that a little knowledge goes a long way in getting the best performance (and life) for your specific application. Like; never discharge below 50%, batteries love to be at full charge when idled, etc. Hope that helps.