The normal size of 33 gallons for G-series chassis of that era means stopping at least every four hours. At highway speeds of 55-70 mph you'll be using 7 to 8.5 gallons per hour.
Larger fuel tanks did not show up in G cutaway until it was totally redesigned as the Express/Savannah with separate body and frame. Tank location changed to meet evolving Federal safety standards, so it is probably not something to retrofit.
There was a larger tank offered for the G-series bare rail chassis, built for panel vans and smaller type-A motorhomes. I don't know if the tank location is the same, or if your chassis might have room to fit the larger tank.
