My mileage is usually in the low twenties in suburban driving and mid twenties on the highway. If I keep it below 70 most of the time I can get a hair over 26 mpg. These aren't VW TDI's; but remember a gasoline Benz of this era wouldn't get over 16 mpg highway, at least with me driving it For me to get over 20 mpg in anything but a 4 cylinder econobox makes me happy.
Things to check for maximum efficiency? Tire inflation, air and fuel filters, oil viscosity (switch to synth?), dragging brakes, injectors, leaky fuel return lines (I can smell fuel under the hood after shutoff) etc. Try running a bottle of Redline Diesel Fuel Conditioner through your SD to clean out the fuel system. Can't hurt.
Also, you don't say where you are. If you are using winterized #1 diesel (most of the northern US switches to this stuff around the end of November or early December) it has less paraffin content and more aromatics to prevent gelling at colder temperatures, and will deliver poorer fuel economy than #2 diesel.

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Bob Roe
Lehigh Valley PA USA
1973 Olds 88, 1972 MB 280SE, 1978 Datsun 280Z, 1971 Ford T-Bird, 1972 Olds 88, 1983 Nissan Sentra, 1985 Sentra, 1973 230.6, 1990 Acura Integra, 1991 Volvo 940GLE wagon, 1983 300SD, 1984 300SD, 1995 Subaru Legacy L wagon, 2002 Mountaineer, 1991 300TE wagon, 2008 Murano, 2007 R320CDI 4Matic 52K, some Hyundai, 2008 BMW 535xi wagon, all gone... currently
2007 Honda Odyssey Touring, 2014 E350 4matic