If you are talking about black/gray water tanks, try using lots of water in your black tank and if you want to make sure the tanks are clean use some Calgon water softener, laundry detergent and finish up with a small amount of bleach to sanitize the tank.
If you are talking about a fresh water tank, just fill it up with water and add a cup of bleach. Let it sit for a couple of hours and drain. Re-fill it and drain again. That should clean and sanitize it.
While the bleach is in the tank, pump some water through your water pipes to sanitize. Make sure you run fresh water through your pump and water pipes after you drain the bleach out.