I towed a Surveyor Select SV264 with my F150 ecoboost and 3.73 rear. I got about 9 mpg at 60-65mph. When I got my '15 F350 with the 6.7 I got 11.2mpg. From my towing experiences since then I get from 15-20% better fuel economy towing with the diesel.
In city driving the eco beat the diesel by about 1.5mpg. On the hwy at 60mph the diesel wins by .5 mpg. At 70mph and faster the eco would win, mainly because the F350 is so tall
'15 Columbus 295RL
2015 Ford F350 with 6.7 PSD
2014: camped 12 nights