I'll add this
at first I was strongly considering doing this same project.
After far too much thought on the matter, and after reading a bunch from very experienced long term RV'ers
I came to the conclusion that i am just as well off to use the money towards buying coffee or something.
I just simply don't care if I get every last drop of stuff out of the tank.... and I realized taht i will never be able to anyway, short of perhaps with a huge volume of water and repeated rinsings.... even then it's doubtful. In the end, who cares! It will eventually break down and come out with some future emptying... and further more I'm not the least bit interested in sanitizing the thing. There's no need.
Use plenty of water and never empty before the tank is at least mostly full, and you'll never have a problem.
And on the rare occasion when I feel the need.... for example if I dump before it's as full as it should be, then when I'm at home and not holding up the line at the CG's dump, I will on occasion fill or partially fill the tank with clean water and dump that as a rinse.
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