Years back I had a 12 gallon tank with no gage, what a pain to check for fullness ! I made a dipstick from a piece of wooden dowel rod. Added one gallon at a time and made a mark on the dipstick. While it took some time, it worked out great. Tank was under a ledge on the boat, hard to see in, but the dipstick turned out to be very accurate, take off the cap, stick dipstick in, then out and replace cap, knew how full/ empty the tank was. Gages are easier, but often not accurate, never had one as accurate as my dipstick anyway.........