My first guess is rust around the connection for the fill hose...
My 2nd guess is a broken/loose hose. Either the fill hose or the vent on the top of the tank.. leaning more towards the vent..
All of those are going to require dropping the tank, at least some, so you can look on top to see where it is coming from..
I have seen some really nasty pics of the top of explorer tanks in the rust belt where the fuel lines coming off the fuel pump were rusty/crusty/leaking.. I have personally never seen that here, but I'm in the desert.
~Mark