RV AC's do not make cold air, they cool by REMOVING heat from the inside air as it is circulated thru the AC unit. Each cycle, 20 degrees is removed. This creates a LOT of condensation, hince the water that runs off the unit..... Having your doors open, Becky is right, humidity is not being controlled. I have seen this in non insulated ductwork also. Not much you can do to fix that, since the ductwork is in the ceiling.