I appreciate the time you took to shoot some photos. I often do the same backflushing with gray water thing. Never more than once or twice. I think it is at least as effective as the built-in tank flusher we also have, and I don't have to screw around pulling out a gray hose. My impression is that this does the job of picking up a few solids that dropped out of the stream.
I'm not a fanatic about cleaning the black tank. My opinion is there is no such thing as a clean black tank. The best thing you can do to clean up any solids is to dump immediately after pulling off the highway, while all the stuff is in suspension. Those folks who sit on a site for week after week, might have more concern about build-up. I dump at home now, and try to get right at it as soon as I've parked.
But backwashing with gray is free, and literally takes 2 minutes, and the gray has to be dumped anyway, so why not? Black water will never enter your gray tank if you do this properly, and you would have to really work at doing it wrong to ever get that.
