Why not have two batteries, if there's room for them. Never hurts to have more than enough capacity. I made sure I had two batteries in my FW I bought two years ago just so I could have a bit more time without having to ramp up the generator. While camping at Grand Teton in June this last two years, we have been able to get about five days of use out of the batteries before resorting to recharging them with the generator. And, if you have room, two six-volts would be great as they have greater capacity, reserve and power than do two 12-volt ones, if I remember prior posts correctly.
2012 GMC 2500HD D/A Crewcab
2012 Jayco Eagle SuperLite HT 26.5 RLS
2009 Crestliner Superhawk 1900
2013 Polaris Sportsman 550 EXP
2008 Polaris Sportsman 700 X2