I believe John & Angela's mileage claims completely ... just look at their ultra-streamlined Class C ("B+"?) ... even more so than any Phoenix Cruiser model.
FWIW, we try to maintain close to 55 MPH with our 24 foot E450 Class C when cruising and it sure is tough ... other drivers keep pushing us all the time with their hurry-up attitudes. We can get "around" 10 MPG at times at these speeds. I had to put non-stock over-diameter tires on it to help counter our E450's 4:56 rear differential ratio, though. Where is Ronald Reagon when you need him.
By the way John & Angela, are you the folks who posted sometime back about removing all the decals from your RV - leaving it a beautiful pure white? (At least your Vanguard photo sure reminded me of that earlier posting.)