My short bed truck also had a small fuel tank. I considered upgrading it, but really didn't have spare payload I wanted to give up to carry extra fuel. I could never pencil out the savings of buying the tank and filling with cheaper fuel rather than making more stops, but it would have been convenient. My OEM tank gave me a practical range of 250 miles between fill ups when towing which would equate to two refills a day for me on long trips.
My current chassis cab has about 500 miles of practical towing range allowing me to refill once a day on long trips. I still make stops more often, but I do see the convenience.
I feel using a phone application, like GasBuddy, is the best way to save on fuel rather carrying large quantities with you.