Truck Repair: Possible Fuel Leak, oil dipstick, gauge needle

Question
99 Freightliner Classic.  Fuel Gauge visibly moving down while driving.  Stopped twice to put fuel in on an approximate 300 mile turn. (140 gallons in all)  Can find no visible leaks on ground, from fittings or under the hood.  Fuel gauge needle does not move around, just down towards empty and does come back up after fueling.  Where could the fuel be going?  Was getting 7 miles to the gallon last week...


Answer
The fuel gauge sending unit seems to be bad --- that would explain the gauge problem ---

7 mpg sounds about right for the truck  - If the fuel is not leaking on to the ground then it is filling the crankcase --- pull the oil dipstick --- is the oil washed down with diesel fuel.