On my '93 V6 3.9L automatic transmission (the same as OP has), I get close to 20 on highway (maybe more), and average at least 16 or more combined. I have kept mine in very good mechanical condition though - never changed the O2 sensor or EGR yet though. It runs so good, I don't think I need to change those yet - hmmm, maybe I do!

I just did a coolant flush on mine yesterday using the Prestone back flush kit (about $4.00) and it is not hard to do. These Dakotas have a great drain plug at the bottom of radiator, plus, the coolant overflow tank is very easy to lift off the side with by pressing a clip on the bottom thereby making it easy to wash the tank. I'll say it again - these are GREAT trucks!

 

I have a 95 Dakota SLT 3.9L V6 automatic and I get like 11MPG. It has 223k with new spark plugs, breather unit, PCV valve hose, PCV valve, distributor cap and rotor, K&N air filter

Could the EGR valve cause this even when I don't have a rough idle, overheating or knocking? $73 for EGR valve!

 

Wow, that's some bad mileage for the v6. As mentioned, the o2 sensor is a major factor. You mentioned the plugs, but what about the wires? Perhaps doing the distributor cap and rotor, too, will help. Are your tires inflated equally and to the correct pressure? Clean out the throttle body and use some fuel additive.

I just recently did a tune-up on my truck and after averaging out 2 different trips, I'm getting 19mpg with both town and highway driving. That is, of course, with me driving like a grandpa and only doing 60 on the freeway

 

Changed my plugs, wires, distro cap, distro rotor, and pcv valve this afternoon, didn't get to do enough driving on it to tell if it's gonna make much difference. The plugs definitely hadn't been changed in a good while. I also bought a fuel filter to install when I can get it up on a lift at work tomorrow. Gonna seafoam it tomorrow as well and see how it goes for the week. If it's not noticeably better I'll swap out the o2 sensor this coming weekend as well.