Gas tank

If you cannot see where the fuel is leaking from, you know we cannot either. You need someone to crawl underneath to look for the source of the leak. The best suspect for a slow drip, only while adding gas, would be a hose clamp on the rubber hose going from the gas cap to the tank. There may also be a smaller vapor hose running alongside the filler tube. That will have hose clamps on it too.

If the leakage only occurs after the tank has been filled, suspect holes are rusted in the top of the tank. That only pertains to metal gas tanks. If your tank is plastic, a better suspect is a cracked plastic fitting on the fuel pump assembly. Typically those only leak once the engine is running, meaning the fuel pump is also running.

Let me know what it takes to solve this.