Black water tank light says 2/3 full after draining

Similar to lanerd, I put a half box ridex and a gallon of vinegar into the black, bathroom and kitchen tanks; divided according to capacity, before the last dump. The ridex knocks things loose, and the vinegar dissolves calcium deposits (most rv destinations have very hard water). My last overnight is about 100 miles from home, so I drain and clean as well as possible. If the rv has been sitting for a few months, I do the same before leaving on a long trip and drain when tanks are full. Works for me!