Hi there RC:
Glad you took the time to post your question.
Since you usually use shore power, I would suggest that two 12V deep cycle batteries would be the best for your rig. You can easily get by with two 12V Group 24 deep cycle batteries.
Regular car batteries are not recommended, since they provide a lot of power for only a short time, whereas the deep cycle batteries can provide steady power for a much longer duration.
You can usually find acceptable deep cycle batteries at Walmart.
For your limited use situation, spending the extra bucks for two or four 6V deep cycle batteries doesn't make economic sense. Besides, the 6V deep cycle batteries are taller than the 12V deep cycle batteries, and therefore are likely to not fit in the existing battery space of your Class C.
(Even in my Class A, the 6V batteries are too tall for the existing battery compartment.)
Cheers and happy travels.