Inaccurate level readings are quite common. They usually electronically sense when liquid levels are covering conductors. This can be inhibited by 'crud' (technical term) coating the inside of the tank. To remove this crud and keep from building up odors and future problems, here's a few suggestions:
1.) The GEO method of tank cleaning and maintenance. https://sites.google.com/site/cbruni.../site/cbruni/v
Calgon Water Softener Powder can be purchased from Amazon. Double the order to get free shipping and enough for you and a friend. Amazon.com: Calgon Water Softener Powder, 40-Ounce (Pack of 2): Health & Personal Care
2.) Add some Dawn dishwashing liquid to the grey tank, half fill with water and take a trip. The sloshing will help cut grease.
3.) Always have water in black tanks to prevent the 'Pyramid of Poo' that can cause waste to form a blockage. The Calgon helps make water wetter and breaks this up.
4.) Only empty tanks when at least half full, this allows a more complete flush. Dumping too often means strong flow doesn't develop and move things out of the tank.
5.) If you have internal tank flush nozzles, use them to help rinse tank after dumping.