You will find many conflicting opinions on this subject. Most of the opinions will justify what choice the individual made. I do not think there is any data which demonstrates 6 volt batteries will perform better or last longer than equivalently sized deep cycle 12 volt batteries.
My battery compartment is relatively small so I picked a pair of 12v batteries that totally filled the compartment after I discarded the sliding tray to make more room. I also opted for AGM batteries because I did not want to check or maintain the batteries. At this point I open the door to the compartment once every year or so just to be sure there is no issue with the wiring or corrosion. I also opted for the best quality batteries with a good reputation: Lifeline. Now my only maintenance or concern is to be sure I fully charge the batteries at least every few days of use.