For the folks that mention to use lots of water, are you on full hook up sites where you can easily dump a black tank? If my family of 4 is at an electric/water only site for 3-4 days, my black tank will be full just from regular use where I only add 3 gallons of water to the tank at the start of the trip.
Those agitation test don't seem to mimic reality also...if you are traveling with water and waste in the black tank then I'd see the agitation but if you are sitting at a site for days there is no agitation.