Well then, there are a lot of things you can do to that Cummins.
The air filter, either K&N or FleetAir is still number one and will increase fuel mileage at $300 or less.
Two, reducing back pressure on the exhaust side is the next economical move. Check with Pittsburgh Power, they have mufflers for about $200 that reduce back pressure which increased mileage.
Three, for more cost, a ported and polished manifold.
Four, Reducing rolling resistance by your choice of tires. You can check rolling resistance at http://www.michelintruck.com/michelintruck/toolbox/toolbox-landing.jsp# Look on the left of the page, fourth selection down is RR comparison, most every brand is listed and all tested the same, the lower the number the better.
And finally, for the Pi?ce de r?sistance, a Pittsburgh Power Box. These things aren't cheap, and if you set it more for power than economy, you will actually lower your fuel mileage, but that MH will run like a bandit! It has settings from lowest power, highest fuel mileage, to highest power, lowest fuel mileage, you can change it on the fly. So if you are going though the mountains pulling a toad, turn it up, on flat land, turn it down and maximize fuel mileage.
Here's there web site; http://www.pittsburghpower.com/
In trucks used 100,000 miles a year, I do all of those; but on a MH, I'd do the air filter, muffler if available for my MH, and the PP Box (Just for fun). At the price of the box, I don't think I'd ever drive a MH enough miles for a ROI, but it'd give me a lot of fun back for my investment.