How much water to ad to black tank when starting out.?

Like you, I put about 4 gallons of fresh water in the black tank after emptying - but I've never used any additives. I've also not had any issues in over four years of full-timing. Several times a year I will use the "Geo method" to clean the tanks: about 1/3 tank of clean water with some dish detergent and borax crystals before hitting the road. The agitation and solution help clean off the level sensors and get residual "stuff" off the sides of the tank(s).

Rob