Close the dump valve, run in several gallons of water, then pour a 2-4 cups of bleach down a drain (depending on the size of the gray tank). Continue running water til the tank fills, then let it sit for a day. This will kill most anything in the tank, but doesn't really clean it out.
Another technique is basically the same but you add an enzyme such as Roebic 57 or Rid Ex instead of bleach and let it sit for a 3 days.The enzymes will eat the greasy residue that clings to the tank walls and sensors.. It doesn't "sanitize" in the sense of killing germs, though.
I'm not sure what the point of "sanitizing" the tank would be, since the very first time you run some water down a sink drain it gets contaminated again. Are you having a problem with odor or clogs?