My method is not too original but I've run the rinse 5 or 6 minutes with the dump valve closed and with my wife watching with the toilet valve open. Every time the tank empties, "stuff" comes out. I use a clear elbow. My unfortunate experience came when I wasn't dumping the tank but was using a "Y" connector to wash the trailer. I was on the far side and heard water running. As I looked under, water was coming from the belly vent. The excess water had overflowed the toilet. You can imagine how I spent the rest of that day!
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