If the gauge is accurate, it would sound like you have a 33 gallon gas tank. I've never owned a 1985 G30, but searching the web shows that 33 gallon was indeed a size used in 1985. On newer vehicles, the size of the gas tank is usually on the tank itself, at least on two of my vehicles that I have actually checked.
We have a 2008 E450 with a 55 gallon tank. We get about 7.5 mpg at the speed limit, so we generally stop anywhere between 280 miles and 300 miles between file ups. If we don't need the generator, we might push it to 325 miles. Where we live the speed limit is usually 70 mph, so we generally stop every 4 to 5 hours on long trips to fill up. Wish we had Strabo's setup since we generally travel about 700 - 750 miles a day.
-Michael
