Right where the fuel lines run.

 

It's tight up above the tank. Get a mirror and a light up there and you'll see two sensors up there. I had to put some sealant around where those sensors plugged into the tank. Otherwise I'd have a gas smell particularly when the tank was full. I just think the rubber gasket around those sensors dries out over time.

 

Also, if you decide to drop the tank for repairing those gaskets, think about replacing your trailing arm bushings as well. The front bolt is much easier to get out with the gas tank dropped.

 

Also, dont plan on dropping the tank until you are near empty.... gas is heavy.

 

In addition to the other great advice, if your rig has over 100k miles I would replace the fuel pump, while you are in there.