My ML320 did between 20/22 around town and up to 29mpg on a run. I was very impressed with it.
The one big drawback with any W163 ML is the prodigious rate at which it eats brake pads and disks. Other than that they are, in my opinion, well built, solid, dependable vehicles. The facelifted ones that is.
If you are considering an ML500 then it will be facelifted.
The trick to getting a good mpg from any car, and I have to say that I always seem to be able to do it, is to ease off the throttle. Especially when approaching junctions. Look a long way ahead and judge what is happening. If the traffic is slowing, then take your foot off the throttle. Let the engine do the braking as it wont use fuel whilst it does this and use the brakes gentley and only when needed to stop or dramatically reduce speed.
If you get an ML500 and drive it sensibly then it will return a good MPG for its size and give you years of trouble free motoring.
Good luck with your purchase.