Before replacing try these:
First try sodium bicarbonate, baking soda. Get a big box in the laundry isle. Mix as much as you can into 5 gallons of water and pump that into the tank followed by another 5 gal of plain water. Let the taps run a bit and then go for a drive to slosh it around. Let sit overnight. If that didn't work...
Try hydrogen peroxide, buy it bulk at Costco or Sam's. It is more aggressive against microbes than consumer grade bleach, unleashing pure oxygen on them instead of chlorine salts.
Use it straight, don't add water.