Sussex Duo leaking air conditioning unit

2011 Sussex Duo. Just over a year ago we found water dripping through the directional air vents inside the van. Marquis thought it was condensation. I took it to a Dometic fitting specialist who was prepared to change the fitting gasket but thought the problem was that the sealant around the unit on the roof had failed. Marquis resealed it. Two weeks ago water is again dripping though one the vents. On inspecting the roof area myself the sealant again looks in poor condition. Part of the problem may be due to the position of the front skylight and the A/C unit, water cannot escape from a section of the roof. Has anyone else suffered from this problem / come up with a method of draining water away from the roof section / got any suggestions. The van is going to AS in a couple of weeks and they will look but I just wondered.
Thanks