To answer your question...lite 18-19 mpg diesel all up at 17800# with boat 11 to 11.7 mpg and I am running down the road too.
To be a little more specific...Are you buying new? The new trucks have loads of power but that takes fuel so shouldn't be too hyped up on getting better or equal for a bigger truck...Buying used? now that takes on a different look...some trucks have done better from year to year with in the brand and across the big 3. So I think what you need to look at is...what are you considering and re-ask the question. Plus find out about what the issues are for that truck, some of us will tell you. Good luck!