I have a 1991 300CE, so exact same engine. My city/short frwy commute mileage averages 18-21. Long trip highway mileage is marginally better at 25. My engine does have a small idle issue that I haven't tracked down yet but reading indicates a possible vacumn leak.

Your slow acceleration might provide a clue. I generally drive like an old lady but on the occassion I stomp on it, it really wails.

glenmore
1991 300CE
2000 C280