I have an F-250 extended cab pickup with the 7.3L diesel and the 6 speed manual transmission. Gas mileage is about 8-9 mpg while towing the boat. Not too much better while towing, but when I travelled together with a friend in a V10, we were stopping every 3 hours for fuel for him and I could have lasted about 4 hours between stops. I do get about 18-20mpg when not towing or heavily loaded. Also, when I got my truck in 2000, the F-250 and F-350 had the same towing capacity. The 350 just had a higher GVWR allowing a larger payload.
Towing with a two axle trailer seems fine. it tracks nicely. I have electric brakes on both axles. Triad really has their sailboat trailers down and needed no adjustments other than routine maintenance. It did take me a couple of trips to get the boat positioned best fore and aft to get the best tongue weight.
