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...
https://marinehowto.com/what-is-a-deep-cycle-battery/
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...