On my previous unit, the line that ran parallel to the tank drain was the overflow tube.
When the tank got full it would gush out there.
If you kept filling, it would eventually come out the vent tube port too (screened part by fill port) and was not a recommended method.
I wonder if the vent screen was sealed by mistake?
Do you think there is a tube attached to the back side of it?
2016 Flagstaff 8529IKBS Diamond Package 5th Wheel
Ford F-Series SCREW 4x4
TST Tire Monitor - Sinemate 3500w Gen.
2016 Nights Camped = 42