From your post I would assume you are pulling a travel trailer. Wind resistance is the big killer, the wieght wont bother the ML. So milage depends a little on what you are towing. If the trailer is not much taller and wider than the truck your fuel consumption will not be too bad. I would expect about a 5mpg drop from unladen, though I have no personal experience towing with the ML, just a guess based on lots of other towing experience. That probably puts you at ~15mpg. Keep in mind that nothing capable of towing that much is going to get "good" milage.
