Short answer – we don't. It's fairly common for black tank sensors to become useless after only a couple of full black tank cycles. At first, this was frustrating as we really thought we'd need to know the status of our tanks all the time. In reality, you just dump once a week or when the toilet “burbs”. When completely full, the water in the toilet will release a gas bubble when flushed that becomes more and more pronounced as it gets more full. We haven't had accurate black tank sensors in over 3 years.
How do you keep your black tank sensors from getting fouled and providing false readings?