I mean, what's going to happen to anything left in that tank? (Down into the hole in the ground and the septic system) so why bother.
Anything left/stuck in the tank and/or sensors will harden and it will not necessary come loose just by adding water and dumping. It will just attract more to stick to it.
And dawn, calgon, water softeners and all the mystical magical treatments for holding tanks leave a film/surfactants on sensors which is why so many people are always complaining their sensors don't work and everyone always reply that they never work anyway. :(
No you don't need a black tank you can eat off of, but paying attention to a few small details keeping it clean eliminates problems down the line. ALL my coaches I have owned had fully working sensors, never any sewer smells up to the day I sold them.