Since I'm now "Full Time" I do not clean the black tank much, I do rinse it out well if I'm planing on a long time between dumps though (I've gone up to 8 days)
NOTE that the sensors tend to show part full even when it's empty and visually confirmed as such.. This is normal.
I replaced "the Flusher" (TM) with a "Tornado" (TM) in the tank, this does a better job of flushing it out.. I will do mutiple flushes.. And to clean the sensors off so they work better I have tried the "RV Swivel Stick" (TM) by Camco and a home made version of the Valterra Master Blaster (TM) Mine is a bit better than theirs for My rig, might not be better for yours. (More rugged design)
That's the only one that has truly worked.
I also use the "Geo Method" I will describe it..
Dump and flush well, add 1/2 cup each Dawn blue (The original) dishwashing liquid, DO NOT USE THE ANTI-BACTERIAL (yellow) formula, 1/2 cup Calgon Water Softener, and water,, In my 36 gallon tank I use about six gallons (I have a handy, dandy six gallon jug)
Drive.. 50-100 miles.
Dump again.. Now when I dump this 2nd time it will show full, NO option here it will show FULL and the reason is that it is FULL.. Of soap bubbles.
The Tornado will knock those down. Or the wands I listed above.
Home is where I park it!