Sounds like you may be referring to one of the vents. They are connected with a tube. This may help you



There's one in the front (towards front of the truck) and one in the back in the same location. This is 26 gallon tank, 4x4 standard cab short bed. Some of the tanks are different

Unfortunately you can't work on any of this without dropping the tank or taking the bed off (may sound easier, but it's not)