Occasionally, people write to me asking how much water softener, detergent, and bleach to use. I outline it in The Geo Method article. It takes a scoop of laundry detergent in a washing machine to wash clothes. It takes a half cup of Calgon water softener to wash clothes. It takes a cup of bleach to wash clothes. RV holding tanks hold WAY more water than a washing machine. RV holding tanks hold WAY more dirt than a load of laundry. You have to use enough water softener, detergent, and chlorine bleach, to clean RV waste water tanks because they hold more water and dirt than a typical load of laundry.
You'll have to figure how much you need for your tanks. However, if you don't use enough to do the job, don't be surprised when the job doesn't get done. Try again using more.