I've heard of folks getting 20-22mpg US in highway use with a very light foot, but I don't think that's a very consistent number.
Our 1995 C36 had a smaller engine, but made similar HP numbers. We got about 16.0L/100km's in combined driving...(that's 15mpg US).
In contrast, my C230 gets about 9.0L/100km's in all city driving, and 7.5L/100km's highway, sometimes better if I manage to keep close to the posted limit... (27mpg/32mpg)
1998 C230 "Black Betty"