CW had a kit to clean the water system. a bit more involved than just bacteria killing bleach. If you have algae etc now in the tank once the chlorine is gone it can come back.
I would buy the kit. ..but i don't remember the steps to clean or what is in it.
Super saturation with bleach will kill everything though but...it is tough as heck to get its traces out but still a great idea. Half a tank of super saturated water on a rough road and lots of braking should get all the corners. While at it clean your gray and black tanks good with bleach. For sure you got some nasties there.