having a problem and need some advice

[BoostingTurboNut]

im working on a buddys truck its an 00 f250 7.3 auto 4x4

when he got it he had complained of bad mileage about 10mpg, so i went and did a full flush in fluids and filters

oil change
fuel filter
thermostat
tranny filter
diesel klien

truck ran great ran at normal operating temp. , tranny didnt look bad.
fuel filter was dark,
after that the truck ran well was making 16mpg lifes good

but two tanks later, i had to pick the truck up , due to worse milage about 9mpg

i just picked up the truck and checked the filter looked clean, has some debris on the bottom of the canister but the filter looked fine. running normal temp, no mil lights, and no miss fire.

anyone have any ideas?