Whiletwo, 6 Volt batteries will work fine, I opted fortwo 12V batteries. The issue that you need to think about is ampere-hours, not voltage. Both types of batteries can provide about the same amp-hrs, so there isn't much difference that way. I have two 12V for the simple reason that if one battery fails, I'll still have 12V power. I can easily disconnect the faulty battery and still have the other battery available to provide my 12V services. With two 6V batteries, you won't have that luxury. If one of the 6V batteries fails, then you have a problem that will need to address asap.
Here is a picture of my battery compartment. Yours should be similar.
Edited by: Hamops