It's caused by the humidity in the surrounding air and being the vent is colder draws it to the cold vent and condenses on it. If the cold air is blowing against the ceiling and making it colder than the surrounding air it will do the same and form condensation on it also. You could remove the vent and let the cold air fall straight down instead of the vent making it blow across the ceiling. Once the humidity gets removed enough from inside the RV it will stop condensating.
2014 Palomino Columbus 3650TH
435AH 12V VMax Charge Tanks, 520 watt Monocrystilline Solar Array, Morningstar MPPT TS-60 CC,Morningstar Remote,Cotek True Sine Wave Inverter,Cotek Remote Controller,50amp Progressive Hardwire Surge Protector
2015 Dodge 3500 6.7L 4X4
2011 CanAm Commander XT 1000