Class C Gas Mileage

We had a discussion like this in an earlier thread concerning the Chevy chassis. I learned that factors that impact mileage include: motorhomes with a bunk up front (they have the aerodynamics of a brick and are thus referred to as one), motorhomes longer than 25 feet (greater weight), and how fast you drive. Before purchasing mine I researched this forum for the mileage I could anticipate and expected mileage around 11 mpg but in reality the best I have ever achieved is 8.3 mpg. The reason for the discrepancy is that when talking mileage folks on the forum are not comparing apples to apples. Sure I hear of folks claiming to achieve 15 mpg with their class C, but it is certainly not one of the larger class C's with a cab over bunk. BTW - I have a 2008 Outlook 31C on a Chevy Chassis with a 6.0 V8 and generally keep my highways speeds to 60-65.