I installed level sensors on both the holding tank and the fresh water tank. The system is made by Raritan (
www.raritaneng.com). The sensors mount on the outside of the tanks and feed to a single indicator panel. Installation involves cleaning the sides of the tank with acetone, and applying parrallel strips of aluminum tape with the sensor module between them. They have been in place for 5 years now, and are working fine.
I was able to work on the holding tank in place, but I had to remove the fresh water tank. It wasn't as difficult to take out the fresh water tank as I thought it would be. Once the tank is drained, it lifts up out of it's "cradle" fairly easily.
We spend a lot of time on moorings and at anchor, and it was well worth the effort to have the tank levels indicated at the push of a button.