1990 Dakota 3.9L V6

I bought this car and immediately did a full tune up (plugs, wires, cap, rotor, air and fuel filter) I was getting 22 hwy and 18 city. 10K miles later (with regular oil changes and new air filter) It dropped down to 16 flat. I adjusted the timing, replaced the clogged catylitic converter, replaced the seized smog pump and tried running a full tank w/o using the AC. Nothing seems to help.

Does anyone have any pointers on where to look next?

 

The problem reads "DODGE" on the grille. J/K... but not really. Dodge trucks aren't known for their great gas mileage.
On average, I get 13MPG city, and 18MPG highway at 60-65MPH (around 16MPG at 70-75MPH).
That is weird that the mileage dropped at the tune-up (where the goal is, of course, to help the truck run more efficently).
There are a few otherthings that can effect mileage, such as oxygen sensors, MAP sensor, and the coolant temperature sensor. If the sensors are bad, the PCM is supposed to figure that out and turn the "check engine" light on... but thats not always the case. There are ways the test those sensors, do a search and you will come up with some results.

 

Check all the vaccum hoses very carefully. If this happened in a short period of time, It's susally a simple problem. good luck