Leaking turbo/intercooler boots is typically evidenced by black smoky exhaust and odd underhood sounds (whooshing/whistling), especially when under load. It may also cause the MIL (check engine) light to come on.
You certain the transmission is going into O/D and locking up as it should? (Under light acceleration it should shift/and lock at about 38-40 mph, causing the tach to drop to 1,200 RPM or so). Also you certain you have no dragging brakes, including the parking brake?
My '99 F250 suffered sudden significant mileage loss from frozen slider pins on both front and rear calipers. Replacing the calipers returned fuel mileage back to when the truck was new.
'99 F250 PSD - bone stock.
My other diesel ride: The "Collosus of Roads": '79 Kenworth K100, tandem axle, 27' dovetail flatbed, NTC290 Cummins, 15 speed Roadranger.
Old tractors: '53 Farmall Super M, '53 Farmall Super H, '47 Minneapolis-Moline UTU.