Considering engine swap for towing purposes

Hey guys, on a tablet so this will be short. Doing a significant amount of towing with my truck. Trailer is a 7x12 v nose with a usual total wt of 5k. It seems to be working very hard on some of the longer pocono grades. Even though it pulls them at 55 65 I dont like seeing the rpms at 2750 and engine temp creep to 235. Even though I can average 10.8mpg towing so I am thinking upgrade. Looking at a 14 2500hd/duramax. A lot of $$$$$ so need some input. Ie Real world mpg on the duramax and normal op temps/mpg for a 5.3.
Thanks in abvance