Cleaning the Blackwater tank: we found a good way to do it!
(While you're still at the campsite, and hooked up to their sewer...
Drain the Black and Graywater tanks as you normally would.
Shut the drain valve, and then hook up to your San-T-Flush or Tornado system.
(Don't turn on the water just yet!
Put about a half-cup of dishwashing soap down the toilet.
Now: turn on the water, and let it run for a while (Pit about five gallons or so of water into the tank...
Dump the tank... you'll get LOTS of soapy water....
When it quits running out: turn the water back on, and give it about another five gallons or so of water...
Dump it again...
Repeat this until you get nothing but clean water coming through the slinky.
It'll also make the term "Stinky Slinky" completely obsolete!