I have been doing a ton of research on batteries for RV's lately. If I'm understanding the information correctly, you should be looking for a simple marine or deep cycle 12 volt battery. You can find one of these at WalMart, Costco, Sams, etc... If you wanted to power your unit off the batteries for an extended amount of time, you would need to do a little more homework. Also, from what I have learned, you want to stay away from typical starting batteries used in cars. They are usually measured in cold cranking amps. They put more power out initially but don't hold a charge as long.
If you are interested in more information, I have many stored links that could help you :) I get a bit obsessive when learning new things
I can't seem to post a link properly. Here is a URL that can help.
www.solar-electric.com/deep-cycle-battery-faq.html