What you are experiencing is condensation. Just like the water that forms (condenses) on a glass of ice water, air will hold water until it reaches its dew point. Dew point is the temperature at which the amount of water vapor in the air turns back into water because the heat of the air can no longer contain it.
What you have with cushions, just like with house insulation, is the temperature at the surface of the cushion is ambient and the temperature at the bottom of the cushion is whatever it is touching. We had the problem with the fiberglass areas the cushions touched.
The temperature decreases from the cushion surface to the surface the cushion rests on. If this temperature gradient passes the dew point for the humidity content of the air, water will form at that point.
One way to prevent this is to place a barrier which will not allow water vapor past it. A sheet of plastic works well. Just place a sheet of plastic OVER the cushion to prevent the moisture from entering the cushion. You are being surprised by the amount of moisture the human body deposits via evaporation into its environment.
Another thing that helps is keeping some sort of ventilation in the trailer. We always have our roof vent at least cracked and one of the windows
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