Gas leaking from top of gas tank?

the filler neck slides in to the tank thru a large rubber gasket. that and the rubber line you mentioned will deteriate thru time. like he said, the pickup also has a rubber gasket between it and the tank. they can also deteriate also, but are thin and seem to last many years.
its a hassel but pulling the tank and inspecting the pickup unit, rust inside tank, possible holes (( many times on top of tank) is a good idea.( unless you are in the dessert).
all this stuff is sold on feebay and thru repubtable new parts vendors. there is a quality difference ( and price) in it all! hopes this helps.