I have a few thoughts on these questions. First, a mid-size truck (M2) should have no trouble in the mountains with a 24' coach pulling along a truck. A lot of M2's are equipped with smaller engines. We had a FLD 112C with a small block Cummins and it pulled our race trailer just fine on hills of all slopes.
In my opinion the M2's don't ride as well as bigger cabs. The Cloumbia Class trucks are Class 8 if I'm not mistaken, just a cheaper model in Freightliner's lineup. The Century Class is a bit fancier. I'm not an FL expert so take my words with a grain of salt here.
My experience with fuel efficiency is that the difference will be small. The big trucks are so underutilized that you don't pay a big price.
If fuel efficiency is big for you look at Volvo engines or Cummins small blocks (M11 for example). We got just under 12 MPG pulling a 25 foot coach and a 6000# race trailer.
One thing left out of your questions: The cool factor. I think that Volvo VN's, Peterbilts, and Kenworth T2000's are way cooler trucks and make better looking conversions. As the drive train is virtually the same between platfoms it comes down to looks, and reliablility. I'm already seeing used truck prices spike from the new fuel regs. Get in early.
Good luck, Tony