As stated before, black tank are notorious for reading incorrectly, even after a good tank fill and flush with an onboard "no fuss flush kit".
Sometimes, it takes a little more physical "encouragement" to get the deposits off the tank sensors. You may want to invest in one of these:
https://www.walmart.com/ip/Camco-Swi...xoCuIoQAvD_BwE
This wand in line with a good amount of water pressure from a garden hose can be placed down and through the toilet bowl valve into the black tank. Let the water blast the walls of the tank by changing the angle of the wand. Just make sure you start with an empty tank.
It's worked for me. I usually do this at the end of each season.
https://youtu.be/xJRvK6rU_-k
For regular black tank maintenance, look up "geo method". It incorporates using a water softener and detergent of your choice. It helps keep the tank clean and reduces smells significantly.