Diesels have lost the fuel economy advantage that they used to have as far as towing goes. Part of this is due to emissions and part is due to the increased hp and torque that they produce. They do get a little better unloaded because of the newer 6 speed and up transmissions.
My 05 Ford diesel would get about 14 mpg towing but also got only 15 mpg empty. My 16 Ford diesel gets 9-10 mpg towing and 17-18 on the highway (slow down to 60 mph and it will get closer to 20 mpg). My 13 2wd SD with the Powerstroke would get 21 mpg doing 60 mph.
Towing a heavy load my diesel will beat my ecoboost by 20%-25% but towing a light load (like a Polaris rzr on a 12' trailer I only get about 10% better. I have taken many trips to Colorado with both the ecoboost and the SD and am always disappointed that the SD only gets 1 mpg better. In my experience if you only tow light loads the diesel has no advantage other than the weight of the truck makes towing less stressful