How does this side compare with the other side? Does the other side have the tank fill as well?
Looking at the above picture, is the dip strictly a dip, or is it a wave? That is, up, down, up, down.... Any other pictures? A wave would suggest excess metal in need of shrinking. A loose depression only may suggest loss of crown that needs stretching. As you can see different situations warrant a different repair.
I would suggest using a long straight edge to see what the panel is actually doing, lay it over slightly off perpendicular so it follows the panel, this should allow you to see if it is a low spot or wave.