Our black tank sensor has never been accurate. With a complete and thorough purge I can get it to reset to empty, but it jumps to 2/3 whenever used. I know about what goes in and track usage and don’t rely on the gauge. The grey tank seems to work as advertised. It’s an electric switch which is known to fail and would be easy to replace if you could get to it. As mentioned, the grey tank is the limiting factor so I don’t really watch the black tank when off grid. The black tank with prudent usage can last my wife and I a couple weeks, the grey tank 4-5 days. Good luck.
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