If you are talking about the 2.1 dual turbo they put in an ML250 Bluetec and GLE Bluetec, its a capable powerplant. Its rated to pull close to 7 thousand pounds. I might consider a used one in a couple years.
The GL has had a 3.0 liter for a decade that is rated to pull a trailer over 7000 pounds. My daughter has one and its a nice engine.

Nissan has a 5.0 that gets 20 mpg. Don't see many of those anywhere. I think Ram and Ford both hit the market for a half ton exactly right. Not a huge market for commercial applications towing over 8k lbs. Folks who need that much power just go to a 3/4 ton anyway. You might as well get an ecoboost if you're ok with 20 mpg. It has the towing power you'd want. The point of the half ton diesel is to tow the same stuff as a normal half ton but get ridiculous mpg while doing it. When you get back to gasser territory with mpg, most folks are either going to go with a 3/4 ton diesel to get the towing capability or a half ton gasser for the lower-cost maintenance.

redchevy said it just right - I would love a few extra horsepower on my ED, but I wouldn't trade a single bit of fuel mileage for it.

I've seen guys getting upper 20's mpg with 4x4 ecodiesel and GDE tune. It's legit

I have a crew cab short bed 4x4 with 3.55 gears. I have been averaging 26-27 mpg on my 20 mile one way commute for the last 21,000 miles. During cold spells I get the 26 or a hair less over a tank and when its 50's and warmer its 27 and change. I don't have the tune yet.

Maybe at 60mph from what I've read. It gets about the same as the 6.7l which confuses me.

If your talking about the ford 6.7L then my answere would be its still feeding the same number of cylinders, much like why a 350 and 305 don't get much different economy.

I will forever be fan of the cummins because its an inline motor. I really wish cummins/ram/epa could have gotten together on an inline 4 cylinder in the 4L range.