I also added the Mopeka Water Sensor to our 1995 fresh water tank. Thanks to Virgil for trail-blazing this mod
For easier installation in my driveway I chose to install it from the CURB side, about 9"" from curb side, and 13" from front of tank. See pics
Pressed the bottom of the tub to ensure no piping in the vicinity, felt good, but came close to the pipe anyways. See pic
Calibrated the sensor for flatness, but my tank bottom is not level, so sensor App complains it is not level.
Measured the tank height as about 18cm, used 18cm for App input. Using a known Jerry can, filled the tank with water. Monitor Panel 1/3, 2/3 indicators were pretty close. My Mopeka Sensor cuts out at about 25% fill, maybe due to the slope of the tank, maybe going more the the back of the tank (away from the drain) would give better results.