The standard probe based tank sensors are frustratingly unreliable. They get dirty and stop working correctly. The SeeLevel system uses a sensor that mounts on the outside of the tank and is not susceptible to the same fouling issue. Depending on the size of the tank it can also be much more granular then the empty, 1/3, 2/3, and full of the standard sensors.
I had to pull the underside panels to repair a water leak this summer so I used it as an opportunity to add the SeeLevel system. I have four tanks (Fresh, Galley, Gray, Black) so I used the 709-4 system with four 710-AR sensors. I used the existing wiring to connect the sensors to the display.