Barring complications from the build-up of gunk, probe sensors and exterior mounted sensors will be able to approximate how full your holding tank is within 25-33%. For folks who primarily use their rigs at RV parks, these sensors are more than accurate enough and serve them well.
For the boondockers and backcountry campers among us, 25-33% is a scary range of accuracy. Those looking for a precise reading of holding tank levels will have to splurge on an ultrasonic system