galvanized steel in gas tank

[forestb]

I have a plastic gas tank that hangs under my bed. The bottom of the tank is bowing upwords in the center but not around the edges. I want to put a pole inside to give it back it's original structure. What I was thinking of doing is putting a galvanized steel tubing inside the tank and coating it with the sealer that you use to line the inside of the tank. I can not find the reason I have this problem. It is vented and when I take off the cap it does not change and it does not get squeezed from the sides.