After speaking to a number of Sprinter MB owners I have learned two things: The mileage is often greatly exaggerated (13 to 15 yes, 18 or better in your dreams), and don't forget that diesel costs significantly more.
Second if you are going to pull a toad stay out of the mountains as they lack the power of the big block gassers. The last guy I talked to pulled a small jeep four down and he said 6% grades are 35 to 45 with the flashers on.... :E
Don't get me wrong, I think the MB diesel is one hell of a motor but less than 200 HP is not a lot. Folks don't buy them to save money as the initial cost is around $20,000+ more than a comparable gasser and you will never save that in fuel.
As always.... Opinions and YMMV. :C