SEELEVEL gauges malfunctioning

[OneReallyOldGuy]

The black and gray tank SeeLevel gauges have been giving me problems for over a year now. The fresh water gauge reads correctly all the time. As the black and gray tanks fill the readings begin to go crazy then they just give up and the display reads “OPN”. According to the owners manual that means there is an open circuit.
I changed both of the green sensor strips, then months later took it to my selling dealer who also changed the green sensors and made sure there were no hot wires near the sensors. No improvement. Last month I was ‘up nawth’ so I took it to the DRV factory and they changed the sensors and the display unit inside the coach. Still no improvement.
I’ve tried a variety of tank cleaners the last being a black tank cleaner by Unique. Flushed the black tank, filled it with clean water, added the cleaner, topped off the tank until the toilets would not flush anymore and took a three day trip away from the coach. No change.
I’ve put a whole bottle of Dawn down the drain into the gray tank, filled it with water and let it soak for a day. I’ve tried a gallon of drain cleaner into a full tank, soaking overnight and the lastly 2 gallons of vinegar into the empty tank when I left the factory and drove 300 miles. Still no change.
I am stumped. The service manager at my selling dealer is stumped. The shop Forman at the factory is stumped.
Have any of you had this problem? So far I haven’t overflowed the black tank but I have learned that when the gray tank is full the washing machine will drain onto the floor. That was fun (he said sarcastically)
As always, thanks for your help.
Phil