I don't have a CLASS-C, but I do drive a duly truck. Thinking about your question of leveling both tires, I would have to answer "yes". You need support under both tires. I have heard of people driving a duly (anything) with only 1 wheel on one side and did it without any negative affect. So, I suppose you could get away with raising only one wheel.
But, considering yours is a CLASS-C, first... you have more weight than an empty pick-up truck, and second, you need more stability in your CLASS-C. Leveling only one wheel on one side, I would think, would cause more "shake" in the camper. It's not as stable as all tires on a solid surface.
For these reasons, if it were me, I'd want both tires on the "lift" side, under something solid. In my opinion, there is still nothing better than treated lumber.