I have an 85 300D that gets about 24-25 mpg driving about 60% highway. You milage sounds a little low. There are several adjustments that will help improve milage and a new air filter and fuel filters won't hurt either.
I also use RED LINE diesel additive most of the time that helps keep your injectors clean,another contributing factor to good milage.
If you have the fuel filters replaced, have them pour RED LINE full strength into the large filter. That really cleans the injectors.
[This message has been edited by Mr. BILL (edited 09-13-2000).]