Since your current bank is at 696 AmpH, and usable 313 AmpH,what is your estimated demand use? Since you have a Sat Dish, you would have a receiver. My DVR uses 4 AmpH. You are probably at 75 to 90 AmpH a day user, conservatively. So your current battery capacity would be approx 3.5 days, before recharge. Your info says you don't have solar, so alternator/generator/shore power provides recharge.
The question becomes, how much power do you need and rate/availability of recharge? It would be my own concern not to carry/possess any more batteries than you need to meet your use. Weight defeats horsepower and MPG.
IMHO, a half a GigaWatt would work. :B
b
