Do you have the GVWR of the 5th wheel or even better what it scales at? That may help folks provide more specific examples of what has worked for them. Also, where will you be towing? Are you going to make regular trips to the Rockies or will you be keeping it in the flats.
The modern diesels are very strong, and with their turbochargers they do well at higher elevations.
There are some good gas engines on the market as well. My buddy full timed for two years crossing the country a couple times in a fifth wheel. He had an F250 with the 6.2 gas engine and it did well for him. His trailer was around 12k loaded.
The new Ram 6.4L looks like a very good gas engine (look at the torque curve) and when you choose gas you often gain additional payload. This engine is new and does not really have a track record, but the numbers look good and a couple of forum members have posted good first impressions.
Good luck,
Stu