I am watching this thread with much interest, as a new Mariner owner.
I recently had a 320 Cruisers, in which I installed four TROJAN six-volt golf cart batteries, for the house, and successfully anchored or docked on islands without shore power for two weeks, with proper input from the onboard Kohler genny, and I took along a Honda 2000, since I already owned it, and it's so inexpensive to run for hours.
Needless to say, I kept the fabulous six-volts, but have not yet had a chance to review what's in the Carver - I only know there are two twelve-volt house batteries in parallel.
Interestingly, my first impression is that there seems to be less room in the Carver engine room for additional batteries, but I'll probably find a way by moving or adjusting the battery trays, if possible.
If you have room, and want to switch to six-volts, you will be most satisfied with the increase in amp hours. Four six-volts will beat two twelve-volts every day of the week. A little more expensive to set up, and you'll need some extra cables, but well worth the effort.
Here's a close up of two of my six-volts. I use the T-105.
And a link to Trojan for all the specs.
http://www.trojanbattery.com/product/t-105/p.s. we boat The Bay annually, lurking around Beausoleil, not far from you, so perhaps we'll cross paths and can compare notes. Feel free to keep in touch over the next year, as I'll gladly share any knowledge I obtain with respect to good Mariner battery management.