Just pulled in this evening after a 5,680 mile tow. Yes, there were some miles without the trailer, but less than 500. Total mpg for the whole thing = 10.5. This includes Rte 14 through the Bighorn Mountains, over the Donnor Pass an CA, a hairy up and down from 101 to 1 in CA, and interstate through Cour d'Alene in Idaho. Pulled my ~8,500 fifth wheel without issue through it all. I generally kept to 5 mph below posted speed. Always could keep up to full speed going up any hill, if I elected.
The manual does not require premium for towing, but does suggest you get 10% better milage if you do use it, which I have found to be true. I generally did on this trip. On shorter tows previously, I would get premium if the cost was less than 10% more than regular. Odd that in CA, although gas overall is higher, premium(ethanol) was only 10 cents more. Here in Minn, the price diff for same is over 40 cents (more than 10% over regular. The computer management does a great job of adapting to whatever is currently in the tank.
Make sure to get the Heavy Duty PAYLOAD package, along with the Max. Tow package. Big increase in payload capability. I'm near it with my current fifth. With a regular trailer, even very big, you would be in excellent shape.