These gauges are notorious for being wrong. There are several upgrades that can be done. 1) Horst Miracle Probes. 2)Sea Level gauges. The Horst has a hood like cover over the top of the probe to help with toilet paper and "stuff" from getting hung up on the probe. The Sea Level gauges are ultrasonic I believe and mount on the outside of the tank.
Short of changing the gauge probes, using a tank flush system can help. If there is not one already in the tank they make a sprayer that goes down the toilet piping and when connected to a hose, the nozzle spins to flush debris off the walls and probes of the tank. I have used both of these with fairly good luck. Of course the one you shove down the tank can get a little messy when removing it.
After a time, you should be able to make an educated guess as to the level of the tank by your normal usage.
Hope this helps.
Goody Two Shoes and the Filthy Beast
2008 Chevy Silverado 2500HD
Duramax and Allison 6 speed
2012 Wildcat 282RKX
You can't make things idiot proof, you can only make them idiot resistant.