If they are working properly, that's not normal and warrants investigation... :)
Try spending more time flushing/cleaning the tank when you dump - with built-in tank flusher, wand or backflush elbow or a combination thereof. We have a built-in flusher and I let it run 10-15 minutes while packing up to head off and the tank bottom looks like new afterwards. Not perfect but reads much better. You can buy a retrofit built-in flusher but needs access to the tanks (no underbelly material or re&re it). You need to flush the crud off the "button" level sensors as best you can otherwise the readings won't be right. It may help to periodically use something to clean the black tank - geomethod, commercial RV tank chemical or in the worst of cases Liquid Drano (our owner's manual says to use it and have a couple of times).
In some cases the factory installs the button sensors at the wrong heights (hard to imagine, haha) and you are SOL from the start. If you can get to the tanks, check the mounting heights.
There is a retrofit sensor called
Horst Miracle probes that can help sometimes. Installed them in a previous TT with excellent results but in our current TT didn't help much.
Tanks come in many shapes and sizes with outlets in many locations as well. Some are easy to flush and clean and some aren't. We've had both and now have the worst of them... :(
The ultimate fix is sensors that mount on the exterior of tanks like
SeeLevel. It's too bad 99% of RV manufacturers continue to use sensors that don't work worth a carp. Many RV-ers just use their sixth sense or wait until the tank won't accept anything more.