Chuck,
Regarding the black tank valve, this picture is NOT from a 2015 model but a 2010. In this photo, look just behind the water filter housing and just right of it and you can see a black pipe angling downward. That is the location for the valve. The black tank is just above the valve. The tank is more visible to the left of the water filter housing.
In our case, and we have lived full time in our Mobile Suites since April of 2011, we have not had any issues with the black tank valve and control cable. On the other hand, our grey tank is the one that is the hardest to operate, although it does still work as long as I work with it.
Terry