I have an E-350-based motorhome that weighs in the neighborhood of 9,000 lbs, and it has the EFI 460, so I'm going to jump in: I get 10-12 mpg with mine, believe it or not. 9-10 mpg while towing a car. Go easy on the gas pedal, and the 460 EFI does better than you'd expect.
Interestingly, the same motorhome was also made with a 351 (I think), and that engine is actually LESS efficient than the 460 - possibly because it has to work harder to pull all the weight, so you're pushing the pedal farther down.
Another word of advice: ever see what happens to an underinflated tire? I found out the hard way recently; I mention it because I'm sure there's going to be some weight on that rear axle. Figure out what they should be inflated to and keep 'em there. Mine have to be at 80 psi, which is difficult to do with a lot of air compressors. Also, keeping them properly inflated is going to help mileage, too.