Perhaps your expectations are a bit out of touch with reality.
You own a 22 year old truck that has larger tires than it was equipped with from factory, likely a tired engine and questionable modifications.
Things that can cause poor economy:
Poor alignment
Low tire pressure
Worn suspension components
Brakes dragging ( stretched return springs in drums, brake calipers sticking, or a brake hose inner liner collapsed, e-brake cable seized on )
Driving fast
Oversized tires
Cool air temperature ( warmer air has less drag )
Low engine coolant temperatures
Wheel bearing / axle bearing issues
Lock up torque converter that doesn't lock
Short trips around town / drive thru's
Catalyst brick melted / partially blocked
Worn distributor or timing chain issues
and a myriad of other possible causes.