Wife's slk 230k does about 250 per tank (until light on) around town with a short commute, but will achieve closer to 400 on a steady long cruise.
It may depend on how hard you drive it, I'd guess I could get my wife's car down to around 200 per tank with a bit of effort!
Common cause of poor fuel consumption is often failed Lamda sensors, which are a reasonably cheep fix. Fault codes?