As far as the back flushing, there was a video posted here some time ago of someone doing a test of how well back flushing compared to using the black tank flush sprayer. In the test he built a clear tank and used one of the external shutoff valves with a hose connection to push water back into the tank. The back flush valve helped clean the tank, but left a lot of stuff. Just wasn't enough agitation to stir up the solids in the tank. The standard flush sprayer head did a much better job.