I get about 21 mpg around town with my 97 E320, though it has a different, less efficient engine than yours (M104). I have yet to take it on a highway-only trip, but I venture to say it would be in the high 20s.
My 104 powered W124 got 29+ on mostly highway travel. I would think the W210 should break 30 since it's more aerodynamic than the 124.