JohnOH327

Either my fuel gauge says full or says empty. Nothing in between. Just filled the tank, gauge said almost full, put 8 gal. in it. Pulling out of the gas station, gauge said empty, about a mile later it was full. Do I need to replace sending unit in the tank? Is the best way to do this, is to pull off the bed? My son's friend replaced his fuel pump this way back years ago. Any suggestions? Thanks