See Level Tank monitoring 

Why does SeeLeveL™ outperform other monitoring systems? The sender is externally mounted on the tank, eliminating sewage buildup that causes false readings on probe-type sensors. With exceptional precision, tank levels are displayed as percentage-full. A wide range of models is available for various applications with up to 7 tanks . 

Tank level sensors can be mounted outside in your water and cleanout bay or mounted inside with your other controls