I do not use the "Tow" option. (when on flat ground) but then my trailer is only 7400lb dry. Also this is my third trailer, the first one was shaped like a brick - a huge wind drag at anything over 65mph. With a 7.3 the best it got was 10mpg and it only weighed 5500 dry. The next time I tried a "V-Nosed" trailer (Keystone VR1 at 7600lb dry) that helped the milage with same 7.3 went up to avg of 12mpg in spite of it being heaver than the first one. Then I got a Cummins 5.9 and the "V-Nose VR1" milage went up again to around 13.5 avg. So on trailer no 3 - with the lessons I learned - I went with a lower profile and aerodynamic shaped 5th wheel. Now with the 5.9 pulling a 7400lb dry 5-er I can get 16mpg if I keep it around 60mph. (15mpg at 70 or less).
My lessons were (on flat ground at least) weigh was less of a factor than wind resistance for me. But now when I get in the hills I have to admit that I do put her in "Tow Haul" just to keep the shifting down to a minimum.
I use cruse on flat ground (not in the hills).
My current rig - You can see the aerodynamic front nose on the 5-er