Second the post above to use Google and decide where to go from there.
FWIW, although diesel, I had a previous truck with a small-ish tank and a big 5th wheel. It was not a problem stopping, as per others, about every 200 miles....but we got an auxiliary tank (125 gallon) anyway. The biggest benefits were: a) if we wanted to stop every few hours we could, but we didn't have to; and b) with about a 1200 mile range between aux and factory tanks, even if we wanted to stop a lot, we never had to fuel during the day and could top off in the evenings after we parked for the night and no longer had to worry about maneuvering through the stations with a 39' 5th wheel on the back.<<<< that was worth the price right there.