Even if they're operating perfectly, they suck.
Using a 30 gallon freshwater tank as an example.
If it reads empty you could be empty, or have 9.9 gallons.
9 gallons is enough for a shower and to flush the crapper, empty not so much.
If it reads 1/3 you could have 10 gallons or 19.9 gallons.
If it reads 2/3 you could have 20 gallons or maybe 29?
And that takes for granted they installed the sensors accurately which they never do. Black and gray tanks are even more inaccurate because they never calculate the placement to allow for the sloped bottom of the tank. On a 30 gallon black tank they usually have the 1/3 sensor located so it lights up at about the first flush.
Install this. https://www.garnetinstruments.com/rv-shop/products/
It works every time.