If I understand correctly the only water coming into the coach is from rain not the drain line.
I would take off the inside cover of the unit. Making sure there is a gasket inplace between the unit and the roof. If so I would check the 4 bolts that hold the unit to the roof making sure they are all tight. You then may want to get on the roof and make sure there is sealant along all edges of the unit.
07 Meridian 36G
Roadmaster tow dolly
Great Wife!! & Max the Frisbee chasing dog.