Sorry for this but I need help with my 1992 K3500

First of all, sorry for this! I know this might be the wrong forum for this but you guys are some of the best people on the internet so I thought I might ask anyway.

I have this 1992 K3500 Chevrolet pickup that's getting horrible fuel mileage.. Somewhere about 7/7,5mpg.

i almost always run it in 2wd and so far the following has been changed..

o2 sensor
MAP-sensor
TPS sensor
spark plugs
sparkplug wires
distributor cap
rotor
air filter
fuel filter
ignition module
both engine temperature sensors
thermostat

Unplugged the AC compressor and changed the alternator (was bad anyway)

No noticeable change at all so far.

Hope you guys can help me since I need the truck for work..