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.
I have the Banks on a W22 and altho it performs much better, it didn't help the gas mileage at all. I'm not a hard driver either. I for the first time in my life changed to Mobil 1 synthetic oil and it made a noticeable difference in the mileage. Runs a bit cooler too. I'll never go back to non-synthetic oil. Next is the tranny to synthetic.
I have an old 33' Bounder with a 454 Chevy engine. I try to run light and take it easy on the pedal. The 3 speed auto is no help(no overdrive). I was looking at Tornado(air flow) and a few other gadgets. It doesn't look to me as if any of them will actually work.