I'd expect around 8 MPG from a heavier and larger Class C. Class Cs should be less aerodynamic than As which doesn't help. I've only averaged in range of 8-1/2 MPG on long road trips when driving the Thor 28A Class C, which is about same size and weight.
I'd like to see a manufacturer build a small Class A a little larger than a Class B to see what kind of fuel economy they could achieve. For now it seems fuel economy isn't a high priority.
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