Perhaps your sensors are fouled up with "Black Tank Debris..."

If you have a San-T-Flush system: try this...
Dump your tanks (make sure that they're empty.
Close the valve, and then put about a half-cup of dishwashing soap (We use Dawn) down the toilet. hook up your water supply to the San-T-Flush hookup, and let it put about six or eight gallons of water into the tank. (Let it run!)
Shut it off, and open the drain valve...
You'll get LOTS of soapy water!
Now keep repeating the process, without adding any more soap.
You'll keep seeing sudsy water coming down the Slinky...
Once the water runs clear: you're done!

Then it's time to see if your readings are looking any more accurate.
Good luck!
This also keeps your Slinky sparkling clean also.