Short But Powerful

I am in a search for a "short" diesel motorcoach that can easily tow 15K up any mountain range in North  America.  Looking to keep the coach 40' or less if possible, but willing to entertain some that are a little longer.  I have looked at several pre-def from 2006-2009 but the list is small.  Now looking at 2010's-2019's.  Any advice would be of help.