Bad MPG Blues

[Elripster]

I have a 79 300D. It's gets 19-20mpg with mostly around town driving. If I drive some on the freeway, I might see 21. At 100% freeway, I might get a little over 22. The manual as well as many a post say I should get 24-27. I have done the following:

New injectors
New Fuel Filters
Use Redline
Use biocide until I can get the tanked steam cleaned and all due to algae issue.
Adjusted valves,
Checked for fuel leaks, replaced rubber fuel lines
Replaced the return injector lines
Can't find any leaks
Was able to check compression on 3 of the 5 cylinders that I could get the fitting into the glow plug hole. (1,2 and 4 I think) All were over 300psi.
Tire pressure is to spec and tires are new.
Brakes aren't dragging.
The engine has 280K on it but has been very well maintained:religoius oil changes, valve adjustments, etc....

The car doesn't smoke much at all. Only when you are really getting on it say getting on the freeway. Reving it in the driveway you wouldn't know it's a diesel except it's sounds like one and it produces a nice stink nebula around it after a few.
It starts great.
It has good power, goes up hills at 80 with the AC on not even floored which is good for a motor with 280K on it. She'll break 100mph.

All and all, it seems to run great. It just sucks down that diesel like it's cool. I'm not sure what else to do? Any ideas?

Frank