Workhorse 8.1 liter poor fuel mileage

What is the total wt of your MH? When people state MPG they don't state the Wt of the MH. I averaged 8.7 mpg over 80k miles on an old 454 eng with a 4 speed trans. On the surface that sounds good but the MH only weights 13,500 unloaded and 15,500 lbs fully loaded. Your's probably weighs close to 25 k fully loaded, which is a big difference. Adding headers,FF mufflers, CAI, did nothing to improve MPG either only performance. Changing to syn oil did not help mpgs either. There are two things that helped MPG one was advancing the timing from 4 degrees before TDC to 9 degrees before TDC. This added about a .2 mpg increase. The other was installing an old vacuum gauge I had laying around. The higher the vacuum the better the mpg's. Also it allows me to stay in 4th gear when climbing a hill by letting of the throttle. I have gotten as high as 12.6 and as low as 6.6 mpg. If I keep the speed down to 55-60 and watch the vacuum gauge I frequently get over 9 or 10 mpg but overall it is 8.7. Try slowing down a bit and your MPG's will increase.