Fuel tank corosion
Your cap should be chrome plated bronze or stainless steel and should not corrode. If it really has try spraying it with WD40, let it soak awhile and use your winch handle to open it. I assume since you said "star wrench" that the winch handle should work. If that doesn''t work, remove the screws and the fill hose and the ground wire and replace it. This is an easy job if you have access to the underside of the deck fill.