Whenever I see one of the 24 foot Mercedes Class C's I try and chat with the owner to get a feel for how they roll. The one and only consistent complaint I hear is that pulling a toad up a 6% grades = 35 to 45 with the flashers on. The last guy I spoke with pulls a small Jeep 4 down and said they had just about given up on the serious mountains unless they cut the Jeep loose and have his bride follow him up the grades.
Having passed more than a few in this mode I wasn't surprised. IMHO: The torque of a big block gasser will get the towing done much better. Yes, the diesels get better mileage but they also burn a more expensive fuel and no, they do not get 18 mpg (13 to 15 according to every owner I have spoken to).
As always.... Opinions and YMMV. :C