Regarding the sensor, I think it is the consensus that the black tank sensor is useless and it is a fruitless effort to try to get it to read correctly. After a couple of trips I got used to how’s many days we could use the toilet before the black tank was full. Also, it makes a different sound, like gurgling, when you flush the toilet and it is near full.
Regarding the smell, I use Happy Camper tank treatment and it works great. It isn’t a perfume that masks the smell, it has enzymes or some other type of science that breaks down the waste and neutralizes the smell.
Here is a link
https://www.amazon.com/Happy-Campers.../dp/B007S0H5DU