Gas Mileage Class A
Banks Power Pack. Significant improvement in both mileage and power. Worth the investment. Your post doesn't say what type engine you have but I am guessing it is the Ford V10 because the GM Vortec gets better gas mileage than that. But not much better.
I had a Banks Power Pack on a 454 GMC 1 ton Pickup and the change was nothing short of amazing. They have a website and they advertise in Motorhome magazine. You will probably also appreciate the sound, not loud but a nice mellow rumble for sure.
Good Luck!