Aw yes, the old 6V vs 12V debate rages.
I went through this already. See my thread on large gauge TC battery wiring in the TCU section. I chose 2 12V SEARS PM-1 RV-MARINE AGM batteries and have nothing but good to say of them. Group 31 size and very heavy = lots of lead and amp hours. Currently in store for $229.00. Eventually we will get 3 Kyocera 140 watt panels and a Morning Star 45 amp MPPT controller. Three panels at out latitude will give us a reliable 200 or so watt solar output for charging. We will also setup a yet to be determined secondary load to take use of excess charge availability. Those items along with the 4 gauge alternator charge wire to the TC batteries will give us all the power we could need. Good luck. I have also decided to install an AGM battery in the truck for what I hope will be better characteristics matching to the TC batteries.