Ford does not recommend 91 octane when towing. It just recommends it if you want a bump in performance over the advertised output and it is most noticeable when towing. However, you do not need it if you are fine with the performance of 87 octane.
Those TFL guys need to learn how cam phasers, knock sensors, and computer controlled injection/ignition work. These ain't your old fixed timing engines anymore. They can adjust to different octane ratings and each octane rating will allow it to advance timing to gain extra power. If you don't need that extra power then no need to buy a higher octane.
Also, if someone believes that an Ecodiesel is nose to nose in performance to a 3.5L Ecoboost then they are smoking crack. The Ecodiesel is less powerful, but considerably better fuel economy. The 3.5L Ecoboost is considerably more powerful, but get worse fuel economy. It is up to each person on whether which is more important to them.
OP, I towed up 9,500 lbs regularly with my old 2012 F150 Ecoboost. Not once did I ever need 91 octane and not once did it ever fall bellow the speed limit when towing even when crossing the Rockies a few times. The only reason I got rid of it is because I needed a bigger truck.