First thing is welcome to the forum.
Second thing is that question will get you a whole gammit of different answers, but basically you'll see anything from 13 - 21 MPG depending on how you drive, Town or Highway, altitude.....
I love my 3500, it will tow almost anything I put behind it, but it is NOT a toyota, not built to be a fuel sipper. I am happy with my milage, unloaded when I drove it from Palmdale california to Ontario Canada (3000 miles or 4700 KM) i got 23 MPG but that is only highway driving no city, cruise set at 65 and left till I needed fuel again, for 5 days. That number is hand calculated not via onbard computer, some people will agrue that but I know what I know, now that it's being used as most people use a vehicle (city and highway) (not including towing) I get around 17. Throw on the fiver and it drops considerably.
Hope this helps but if fuel economy is a consideration you might want to buy somehting else.