Rigid and I think Milwakee make cameras that work through a cable so you can insert them into dark spaces like walls, tanks or sewer lines to see inside where you wouldn't normally be able to. They really are not that expensive and you could use that couple with a flexible claw tool to find and retrieve the cap. It would be a lot easier then pulling the tank I would think then if you didn't want the camera once you were done you could give it to me

(actually, you could probably sell it for close to what you paid).