Saltwater Sportman just had a good story about fuel tanks.
Most alumimun ones last between 10 to 15 years.
Sounds like you have a seam ( weld ) that is leaking
since you lose gas when over 1/2 full.
If it only happened near full - then it could be the fill or even the guage seal.
E-10 fuel caused some gasket materials to dissolve - guys fearing a bad tank were relieved to find just a new gasket fixed the trouble.
Gas vapors are very dangerous.
One thing to try if you are handy. You'll need an air compressor and an old car/truck inner tube.
Far away from the boat - run the compressor to fill the tank. Wheel it near the boat. Cut the valve stem from the inner tube - leaving roughly a 5-6 inch circle of rubber. Remove the fill cap and place the rubber over it and add some air. The idea is to GENTLY put positive pressure in the tank to hopefully hear / see the leak. You might need to plug the vent.
You might get lucky and it be the guage gasket or a split hose at the tank.
Losing a few gallons going around the block sounds like a split seam though.
If it is the tank - sorry to say - major mess. You'll need to remove the gas and then the tank.
At 9 years old - IMHO - better to replace then repair it.