I had a clogged black water tank also in a coach we had a few years ago. It was caused by the previous owner having a couple broken tiles replaced. The idiots that did the tile replacement cleaned there tools off that had grout on them in the bathroom stool using the spray wand. Over time all that grout hardened and settled at the neck of the black water drain valve. I removed the “y” fitting that holds the gray and black valves and nothing came out until I used a stick to loosen up the clog then oh boy what a mess. I tried using the sewer drain hose to catch most of the “stuff” but I was wearing more than I was able to control. Needless to say it was a good thing I didn’t know who installed the new tile. Might just mention that during this entire episode my wife was watching from a reasonable distance and couldn’t control her laughter.
