Been doing this with the last 3 RV's. Do this along with your regular tank chemicals after each dump.
The Recipe:
Dissolve two (2) cups of the water softener in a
gallon of hot water. Then, pour the solution down the
drain into the empty tank. Use two cups of softener
for each wastewater tank in your RV. The tank's
drain valve should be closed otherwise the softened
water will just drain out. Then use the tank(s)
normally until it is full and drain it normally. Add a
cup of laundry detergent to the black (commode)
water tank at the same time you add water softener.
This will help clean the tank. The gray water tanks
should already contain soap through normal use.