I always recommend going to the 350 instead..very little difference in cost or ride and you get lots more payload.
Mileage: I gave up worrying about it when I realized that towing a tall, heavy trailer is going to use a lot of fuel..this makes RVing and camping more relaxing.
Mileage 8-12 depending on lots of factors.
"If we couldn't laugh we would all go insane."
Arctic Fox 25Y
GMC Duramax
Blue Ox SwayPro