The best way to increase mileage for free is back off the speed and jack rabbit starts. Yeah, I know people behind you make some feel like they have to "keep up", but you aren't driving a sports car.
Slow down and enjoy the ride. My truck is bone stock and will stay that way.
2006 GMC 2500HD CCSB 4x4 Duramax/Allison, Titan 52 gallon fuel tank, Prodigy Controller, B&W Companion Hitch
2010 Jayco Designer 35RLTS, Cummins/Onan RV QG 5500 EVAP, Progressive Industries EMS-PT50X, TST Systems 507 TPMS, RV Flex Armor Roof