I think my gas tank is leaking

Looks like a rotten tube to tank seal to me, and it's a lot easier to get to from the top inside the trunk.
Just pull the boot and it'll get real obvious.
The seal is easy to replace (just pull the fill tube out and pop a new seal in and put the tube back in.)
A little vaseline or grease helps to get the neck back in.

Orrrrr if you just need something to do go ahead and pull the whole tank without checking the seal.