But some of these can damage toilet seals and valves. A safer solution are chemicals that are specifically designed for use in RVs. You must use specialized toilet tissues. Maybe in old recirculating toilets. But now just buy the cheapest brand of regular tissue, and it will work as well.
Black waters smells worse than gray water.
Not necessarily, because we seldom put chemicals in gray water. The soaps and detergents tend to kill off bacteria in the gray water tanks. No, that's not okay, and it's illegal. The effluents and chemicals will permeate the ground and invade water supply resources. Only by having a substantially full tank can you ensure that it will properly empty all solid wastes.