Fuel mileage

I have searched for answers in various forums but most are out of date or full of trolls/debates. We are looking at purchasing a heavy duty truck for towing a new fifth wheel. The fifth wheel will be on the lighter side. I just would like simple answers - what fuel mileage do you get with a heavy duty gasser or if you drive a diesel, what mileage do you get while towing. No debates please, just answers.