I had an ML270 CDI for nearly 4 years. My overall mpg was c.25mpg over about 38k miles. Towing, I obtained in the region of c.22mpg but in windy conditions, I have seen 19mpg.
I now have a ML320 CDI. I haven't towed much with it yet but my towing average is around 20mpg. Solo, I can obtain about 30mpg on a reasonable run. I am currently trying to obtain 600 miles from my tankful of diesel - the tank is supposed to hold 95 litres but twice, I have put in 96 litres when refuelling. At present, I have done 560 miles but they have virtually all been country miles with very little townwork.