There are many reasons for sub standard milage. Most them are reasonably cost effective to check out. Basically develop a list for yourself to systematically check out. It is basically a tune up type of deal with a few things thrown in.

I can only suggest what I would do if I had a 300d turbo producing say 20 mpg on the highway.

Make absolutly sure the odometer is not hanging up first. Not uncommon. As already pointed out on this thread.

Check for fuel leaks and fix any found.
Make pretty sure a stream of air bubbles are not being processed through the primary filter.
Do a compression check.
Check fuel feed pressure at the injection pump with a gauge.
Check drip timing on injection pump and chain stretch. Check fuel pressure before doing the drip test. They interact to some extent. If the fuel pressure is far too low I think effectivly the pump is still retarded after the drip test.
Check valve clearances. Before the compression check or if any very tight valves with no clearance are found do it before and after adjusting the valves. You want to know what the cause of the low fuel milage was.
Pull injectors and have them checked.
Make sure air filter is pretty good.
Before any of the above verify brakes are not dragging and tires are at a reasonable pressure.

If all these tests do not uneath the problem consider a cheap injection pump from a pick and pull yard. You can stop the testing at any stage the problem is found of course. The injection pump is not totally speculative if all the above is covered first. The pump should be really cheap as there is not a positive certanty the old one is bad just a suspicion it might be at that point. Never pull a dry pump always one with fuel still in it. Just less risk.