I am not sure of the exact mileage on the 1200 but it is right around 15 mpg. Keep in mind the motor is never going below 4500 rpms.
For comparison when I raced a Showroom Stock Miata, the 1.6 motor never went below 5000 RPMs and returned a whooping 9 mpg.
Our current price for gas is 2.35 a gallon, my current tow vehicle (1990 Ford E250 5.8 V8 camper van) returns 12 mpg on it's own and 9-10 when towing the 1200.
As for that old F350, when I sold the Volvo rally car the buyer paid extra to have it delivered some 230 miles from me. On the way back we were in a hurry so I started driving 75 mph and it managed 4-5 mpg.
Now back on topic; I would be curious to see what the difference in fuel mileage is between bike carbs and other types. Based on the RPMs the new motor picked up at the end of the longest straight at my local track I believe the A15 is putting out about 110 HP to the rear wheels. The A12 with 80 HP to the rear wheels, got 20 mpg. When I had a standard A12GX I got something like 23-24 mpg.

Tom