So some quick numbers from the West coast trip...
2010 trip (from Houston to Brea, round trip) with about 250lbs of gear, 1 child passenger, fiberglass camper top (cab high)...3,222 miles, 138.3 gallons used, 23.3mpg. Average refueling distance = 292.9 miles. Average refueling = 12.57 gallons. Partial belly pan, 1st gen side skirts, dropped air dam. No grill block, rear diffuser, rear wheel skirts, or rear skirts.
2015 trip (from College Station to Brea, round trip) with about 300lbs of gear, no passengers, aero cap and aero trailer (about 350lbs combined)...3161 miles, 117.7 gallons used, 26.86mpg. Average refueling distance = 351.2 miles. Average refueling = 14.7 gallons. Full belly pan, rear diffuser, 4th gen side skirts, gap covers, rear wheel covers, rear skirts installed.
I took a major hit on mileage on the 2015 trip while I was in Phoenix, El Paso, and Brea as I was out gitting parts with the loaded trailer attached while driving on the city streets.
Best mileage fer the 2015 trip was from El Paso to Junction TX at 31.06mpg (65mph with mirrors in their normal position) while the 2010 trip was from Ft Stockton to San Antonio at 28.24mpg (60mph with mirrors folded in).