Cool demonstration!
About the residue on the tank sides: Was the tank washed prior to experiments? It may be that the first test with the 3" hose allowed the water tension on the sides of the tank to be relieved by the flow of particulate across the sides (scrubbing pink micro-ish). The following tests, especially the rinse cycle would have relieved even more water tension to the plastic sides.
As long as your willing to run more tests and in the interest of waste tank science, maybe using additives like the Geo method, enzymes, and RV specific tank cleaners could be a series of tests.
