FYeah, I can't understand the need to "clean" the black tank every dumping. My procedure: dump black, close black valve. One 2 gallon pail of water in black tank while dumping grey tank. Dump the 2 gallons, then another 2 gallons in for slosh factor while driving (not true 2 gallons, maybe 1 or 1.5). i'd freak if I got stuck behind someone doing the 15 minute flush on a Sunday afternoon before the drive home!
It liquifies, it flows. I know when to start using the campground facilities, the vent pipe is under"water" so a flush lets a bad smell up into the bathroom. 2 people, 20 gallon tank...3 days.
Ralph