Gas Tank Issues

I have noticed gas fumes in the rear compartment area. (I have an outboard.) I replaced the gas and vent hoses last year. They were beginning to dryrot. But that did not get rid of the fumes. There is no evidence that the tank is leaking.

It's a 60 gallon tank that is 20 years old. A mechanic I talked to said I might need to replace the tank.

Any thoughts or suggestions??