How many floors/levels does my black tank have?

[Henry Claeys]

1) I suspect that my black tank has a problem because I discovered,
after 3 weeks of use, that the drain valve was not closing completely.
(2) What confuses me is, even after apparently emptying the black
system, I still see water when I look down the hole through the
commode. Why is there water in the tank below the commode even after
emptying the black tank? I never used to notice the water until I
discovered the above described drain problem. Thanks.
Henry@... 1979 FC 35, McAllen, Tx