I make it a point to chat with MB Class C owners whenever we are traveling. Have spoken with more than a dozen over the last year. They are always as amazed as I am at the exaggerated mileage claims floating around out there. Every one I have spoken with says 16/17 is doable but 13 to 15 is more of a real world number, above that... not so much. Also every one I have spoken with say they are great engines, most absolutely love their rigs but when it comes to towing.... again, not so much.
I agree that you will recoup some of the initial investment (and that is a very big number), on resale but significant savings from increased mileage just is not in the equation unless you are putting on far more miles than most folks. There is good and bad to be found in any rig. The MB's give better mileage but give up power and payload when compared to the big V-8's/V-10's. You just aren't going to get both in any rig.
As always.... Opinions and YMMV.
:C