Dean, I have an 81 PuP. At 65 MPH I get 34.0 MPG. THis is with the tailgate down running either DF2 or Bio-diesel. As for you the FIRST thing to look at is the air filter. If it has any crud on it give it to the trash people.
Second. Tire pressure. Look at the sidewall, inflate (COLD) to the side wall PSI.
You mentioned the fule filter was replaced about 1000 miles ago. You should be OK with that. Fill the tank to the EXACT same place for each refill and calculate your milage.
The C223 is a tough engine and it will last a very long time with clean oil, air and fuel. NEVER exceed 3500 RPM. The engine will most likely explode (this results in VERY poor fuel milage).
I live in Northern VA and if you are near by I'd be glad to stop over and take a look for you.
Or you can come over and pull your vehicle over my oil pit in my garage.Yeah, a real nice thing to have)
Regards!
Eric