I have a '95 E420. Over the last few years my mileage has been about 15mpg around town and 22mpg on the open road. If you are looking for more than this, I think you may be off base.
Anziani

'95 E420
'87 300E

In a 94 E420 and a similar environment to yours I get 21 city and 29 highway, working at it, but routinely. At 15/22 I would suspect your car is not running properly.

Deanyel,
I would like to hear from other owners of the E400-420 series. My mileage calcs have been with multiple tanks over a long time frame. I know the V-12s get about 20 MPG on the open road. Jim Forgione's S-500 gets about the same mileage as mine. (from his website)
My E420 runs like a dream. New harnesses, thermostat (never goes over 100C), new air sensor, recent plugs, caps and rotors and O2 sensor. Running Mobil 5 (10?)-40 syn oil. and installed Jim Forgione's chip modification. Just got back from a 600 mile trip averaging between 70-100 MPH with an average of 22MPG. I have never had a Engine Light (except before the chip modification) in the last 3 years.
I find it hard to believe that you get 21 and 29 respectively.
Anziani

'95 E420 112K
'87 300E 203K

I've done it consistently with 4 diffferent 124/119s over a period of about 10 years, except 92s get a bit lower mileage, which is reflected in the EPA ratings. I've always been able to beat the EPA ratings, even the old ones, by 15 to 20 percent in all brands and that's all that is. I didn't know you were driving a 100 - I can see 22 mpg at 100 mph. E420 mileage isn't comparable to any other MB V8. It has a 2.24 rear axle ratio, I believe the tallest ever installed in an MB, and it's a light car at least compared to 140 bodies.