As with most vehicles: slower is better. In any RV you have quite a lot working against you: aerodynamically its a brick (even with that nicely shaped windshield it still is essentially a square brick going down the road), there is a ton of weight (we weighed ours and it came in at 11,500 lbs with full fuel, full propane, full water, empty waste tanks, and the 3 of us in it), and a mighty thirsty V-10 engine.
So far for our trips we've been getting around 9 mpg but I tend to go 70-75 mph and we have the generator running the overhead A/C. Going slower (65 or slower) and not running the genny I'd bet we may be able to push 11 mpg.
Even though this is our first motorhome it is our 3rd V-10 engine. Somewhere between 5,000 and 10,000 miles the V-10 will usually pick up 1 or 2 mpg as it loosens up. Which means a well-broken in Axis/Vegas with about 10,000 miles on it going 65mph down a level freeway with no wind should net you almost 12 mpg..if you're lucky.
(The best mileage I've ever got with the V-10 was with our last truck: an F-350, had a custom tune on it for mileage...I got 14 mpg.)
Speaking of tunes; I may put one on our Axis after it has a few miles on it. Even though its already pretty zippy; a tune make make it feel even more spright (given what the tune did for our F-350).
One of the best tuners available for the V-10 is from 5-Star:
(No affiliation with them other than a happy customer. The guys over at ford-trucks.com swear by them.)