You are doing what most folks do. I never rely on my black tank level monitor since always show 3/4 full.
Might add .........consider filling:1) black tank through the flush connection with 15 gallons of water metered by an in-line flow totalizing meter, e.g., $24 P3 P0550 Water Meter, prior to use AND 2) longer than 3 minutes to ensure sufficient water is added to adequately rinse the tank (turbine helps to confirm how much water you are adding during the rinse cycle).
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