I am still not giving up.
I have learned that the "Mirus" tank level monitoring system by Touchsensor is basically the same as the Garnet system. Both use something akin to capacitance causing a current change over a pre-determined resistance. Both use sensors that are stuck on the outside of the tank. In other words, no tank penetration.
To test the system on our coach, all you need to do is put your hand on or hovering over a sensor and with a helper watching the panel, you can watch each light come on as the hand is put over the different levels.
That leads us to what is likely causing the problem if lights are on and the tank is empty. Plain and simple grime on the inside of the tank. It has to be pretty thick. In my case, I can't get a wand with high pressure spray to clean those spots. By the way, my lights are on the grey tank which is unusual. Black tanks have obvious stuff that can cling on the sides. Also, The black tank is usually kept closed, but many of us leave the grey tank open while parked in a camp ground. Wrong - that lets the tank dry and really leave soap scum or whatever on the tank wall. So it seems both tanks should be kept closed unless you are draining.
A tech I spoke with says he usually puts something like "Pure Power", which is an enzyme type cleaner. you leave it in a tank full of fresh water for a couple days. It eats the grime off the walls of the tank.
Okay, it's worse than you can imagine -- he says add four packs of dry yeast and add warm water to fill the tank. Let this sit for c a couple weeks. He says it smells so bad, he parks a rig with the yeast in it, as far out on the back lot as possible. He says this method does not fail. You can use these cures in both grey and black tanks.
One last thing, the folks at Touchsensor use a special two sided pad that conducts electricity. You can get those by calling them. You have to use a sharp putty knife to remove the sensors, or use the Oreo method if the stick on tape is really cured. The Oreo method is twisting the sensor, just like you do to separate the cookie. Then clean the sensors back with your fingers. It's a bitch, but you can roll up the sticky stuff. Then armed with a new pad, stick one side of the sensor and slide it around on the tank surface, AT THE SAME HEIGHT, until you get the correct reading.
I'm using the degreaser in the tank first, but will have to be home to use the enzyme methods. Still in an RV camp and are using the tanks. But keeping them closed to make sure the tank walls keep getting wet.
I have spent many hours coming up with this dissertation. Now, I have all the ingredients and my fingers crossed.