I have a 2001 ML320 and I regularly get 21-22 mpg driving around town. On the highway I get 24 mpg.

You are correct that later model MLs don't get as good mileage as they are heavier. Also a big factor on how well the vehicle will do in the long run is how it is treated for the first 1000 miles. If you gun the engine alot during this period, ultimately engine performance is going to suffer.