I am a big fan of the "Geo Method" using cheap laundry detergent and a water softener/booster (Borax is the cheapest) along with 4 or 5 gallons of water to create a cleaner and somewhat slippery surface inside of the black water tank. No other drop ins needed.
Other will disagree, and I feel they are probably correct in their beliefs for their particular rig, because every toilet, black water tank design and size, air vent, drain configuration, and combinations thereof are not 100% exactly the same.
But it is cheap alternative, certainly worth a try, and it seems to work great for many of us.