F250 diesel fuel tank

[genecurp]

Boy, crawling under the truck is something that can be surprising.

I crawled under the check bolts on the hitch rails to the frame, and discovered this long black thing that is on driver's side and runs front to rear, beginning behind the cab. I take it to be the fuel tank. It was "wet" and oily. Did not smell of diesel fuel but was of that consistency. On top of the tank, when it went under a frame componet, was sticking out half a round gasket, several inches around (dia. 6 inches approx.)

Anybody got a clue what this is and what needs doing?

Thanks and Merry Christmas. This forum is the first place I turn when I have a problem. Thanks again.