I think it's a small (but growing) crowd here on CK5, so you should rely more on advice from expedition portal.
I've never seen 6Vs on trucks, just RVs. What will you be powering and for how long? In a real RV you may boondock for days, but an expedition vehicle tends to move every day, lowering your capacity requirement. Plus you really don't spend much time "living" in the vehicle so that lights are on for hours, there is typically no water pump, etc.
My setup is just a stock-type starting battery and then a cheap marine battery in the Aux tray. This has a separate small fuse block to run the dome lights, PC, CB, GPS, stereo system and refrigerator. I've only camped from it in the summer when we have like 17+ hours of daylight and I've never actually sat in the vehicle watching movies at night or whatever, so there's never been any real stress on the system. If there was, the starting battery should be fine, so I can just start it up and charge. The fridge runs 24 hours on it and never seems to make a difference. Now if you ran a thermoelectric cooler instead of a compressor fridge, that would require a lot more power.