We are having similar issues. You want to accurately know the level of your holding tank, and so do I.
Om my boat I have an automated system that senses sewage in the tank and automatically triggers my onboard type 1 MSD to process that sewage.
The problem I found with the system is that the pressure type sensors the manufacturer used are inaccurate. The manufacturer knows of this issue and as good as they are, they do not have a resolution.
What I have done is to order a few ultrasonic fluid level sensors that I'm hoping will solve the problem. Here's a link to the user guide/brochure for them.
I'll let you know how they work.