I've been doing a lot of research, getting ready to install a new house battery bank. There are so many things to consider - Flooded vs. AGM vs. Lithium; 6V series vs. 12V parallel; manufacturers and brands; etc.
While doing all of this research, I found a lot of info about "Marine" and "RV" batteries and how their performance can really vary. I've also gotten advice from a lot of people about going with 6V golf cart batteries connected in series (coincidentally, this is the setup I already have - what came with my class C).
This article from marinehowto.com is the best I've found, explaining what "deep cycle" truly is - it breaks down the differences between true deep cycle batteries and those that are only labeled as such...
It does not necessarily tell you whether 6V series or 12V parallel is better, but when you see the cross-section of the golf cart battery next to the "Marine deep cycle" one, you can see why so many people go with golf cart batteries. A true deep cycle 12V battery would probably perform as well as golf cart batteries, but you have to make sure you're getting true deep cycle
For what it's worth, I haven't yet decided which way I'm going, but I'm leaning toward sticking with 6V in series, since I wouldn't have to change anything else. Well, I did order a replacement for my infamous Magnetek 6345 converter - orderd the PD 6300 kit (with 3-stage charger) from bestconverter.com - and I'll probably install a PowerPulse desulfator.
I think I'll buy the batteries directly from Dyno since I'm fortunate enough to be in Seattle, where they're made. The main hold-up is I'm still trying to find any data that tells me whether AGM is worth it - mainly for the deeper discharge tolerance...