On some units the shower/tub (grey) runs into the black tank.
Somewhat uncommon but it does exist.
Another possibility is... most units use a common sewerage dump port. If the grey dump valve is open and the black dump valve is partially open or defective, grey water could theoretically be getting into the black tank that way. Slim chance but a possibility none the less.
2016 Flagstaff 8529IKBS Diamond Package 5th Wheel
Ford F-Series SCREW 4x4
TST Tire Monitor - Sinemate 3500w Gen.
2016 Nights Camped = 44