If you plan on boondocking a lot, I'd go with the 6 volt for the extra amp hours, but if just traveling and using shore power, 12 volt is fine. If you go with 6 volt, you want 3 or 4 of them, cause if you only have two and one goes bad you got nothing.
When I replaced my two 8D batteries, I went with 2 8D AGM batteries. When I want power, I want it right now and don't want to worry about watering them! Also wanted them to last a very long time. Don't need to be replacing them every couple years. Now I can park three nights in a row and not worry about running out of power to run the furnace and my breathing machine.
โAll journeys have secret destinations of which the traveler is unaware.โ