We have a control panel above the kitchen range that has several gauges, including two for the holding tanks. When a rocker switch is pushed next to the gauge, the probe is activated and the needle can go to either the empty, 1/4, 1/2, 3/4 or full marks.
On our '74, the wiring for the black and grey water tank probes come out of the inside wall over by where the water hook-up is located. Since I was doing a Full Monte on the back end (it's a rear bath) I had to drop both tanks. I pulled the probes and cleaned both the sensor and connection ends. The ring terminals on the wiring were badly rusted as were the connecting screws. I cleaned up the terminals and used new stainless steel screws to connect them.
When we tried the system out on our first camping trip last week, everything worked. The gauges are not real precise, but they do give you an idea of where the water level is. It is a simple system, there are no moving parts, but the electrical connections need to be good since it does rely on conductivity to operate.