I went through a similar thought process and was looking for a KW or a PB with the Cat C12. When all was said and done the smaller motors don’t appear to provide enough (if any in some cases) fuel mileage advantage to warrant seeking them out. Even 2,000 lbs one-way or the other doesn’t seem to affect the mileage that much. I think an even bigger influence is how many miles a year our really going to drive and what is the real payback. At fewer than 20K miles per year, we decided not to worry about the last 5 percent and go for the bigger more readily available motors.
We ended up building a 41ft 2001 model T2000 with the N14 - 460hp 10sp Autoshift. We weigh less than 40,000lbs with our garage loaded up full of motorcycles. We get almost 9mpg cruising at about 65mph in the Midwest, which is to say not in the mountains. We will know more about mountain performance this summer.
T2000 Complete (but never finished)