A pair of 6v batteries have more than enough amps to provide genset starting power. Just make sure the genset cables are wired to a series pair, for 12v rather than 6v.
12v vs 6v is mostly a cost decision. You can get excellent 12v deep cycles, e.g. Trojan or US Battery brands, but they are pricey. A 12v marine battery is much less expensive but also has a much shorter useful lie (About three years).
Total Amp Hours depend on the size chosen, not 12v vs 6v. A pair of Group 27 or 29 12v's will produce the same 220-230 AH as a pair of 6v GC2 "golf cart" batteries.
So, buy the size (amp hours) you need and the quality (life time) that suits your budget. I've written an article on How to Choose a Battery for Your RV - it is available at Choosing An RV battery
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL