We had the exact same problem with ours, turned out it was mice. they had built a nest in the overhead A/C ventilation runs and then proceeded to then make their own tunnels. Every time we turned on the A/C it snowed in our trailer. Finally I took out all of the louver vents and took down both A/C shrouds and ran a shop vac hose as far as I could reach in each vent opening. Sucked out a big nest in the center of the trailer. They had made entry from going behind the front cap and climbing up to the top and found a entry point that was not sealed from the factory. I had fresh cab, dryer sheets, moth balls, etc. did not even phase them. Finally had enough, canceled my agreement at the storage lot and moved the trailer to a under ground storage facility. No more mice.
2016 Ram 3500 SW CC CTD
2016 Rockwood 8312SS