If I understand you, you currently have two batteries, (I do not remember if you listed the amp hours?), and use your generator several times to dry camp for 3 - 6 days at a time? By simply adding 2 additional batteries, I do not think you will double the time between charges, and as pointed out re charge time would go up. 4 six volt batteries would extend the time between charging, I would also invest in solar panels to charge those batteries during the day. That option would give you the most power and greatly reduce the time you would need to fire up your generator. Definitely on the pricey side, but since you do dry camp a lot IMHO that would give you what you are looking for. Good luck.