I would recommend installing 2 6v golf cart-type batteries. There are a few brands out there but one that I use and would suggest is Trojan. I have 2 T-105's but you can go a step or two up if you like and get into the T-105 plus or the T-125's --- better yet. They are, in my opinion, a very good quality, deep cycle RV and marine battery which will provide you with increased amp hours over conventional 12 v batteries.
Visit their website for information on these and other Trojan batteries (
www.trojanbattery.com)
They also have some excellent links and information on battery care and usage --- a worthwhile and informational user's guide is also on this site.