While it's not all that important for your question, it's worth noting that the Axis is a class A motorhome (built on an E-450 bare chassis), rather than a class C.
I would expect around maybe 8 mpg on the highway, depending on speed and conditions. The chassis and engine should be pretty reliable; the E series is relatively bulletproof and has been around plenty long to have the worst of the kinks worked out. The Ford V8 and V10 engines do rev high for truck engines by nature; it's not going to be sedate sounding on hills or when accelerating.
Beyond that, I'm afraid I can't be too much specific help.