You need to know the total amp hours you're going to be pulling for those two days, then add to that about %50 and build a battery bank that can supply that much power. You don't want to deeply discharge even deep cycle batteries (deep cycles have a higher tolerance for discharge, but they can still be damaged if you drain them too far) so you need to know how much power you need to supply.
As previously stated, a bank of 6v deep cycles will provide many more amp hours than a 12v, and they typically have thicker plates, deeper cases, etc. which will translate to longer life.
If you would like to learn more about 12v systems, I recommend the book "Managing 12 Volts: How to Upgrade, Operate, and Troubleshoot 12 Volt Electrical Systems", or the website
http://www.marxrv.com/12volt/12volt.htm- Randy