We just had the same experience with the under side of the matress being wet. There was also
lots of moisture at the side windows and corners of the overhang. Turns out we had not been cracking the vents open at night when the furnace is on. We even had those vent pillows stuffed in there just to make sure the heat stayed in. Condensation was the gremlin. A friend of ours, who is a full timer, pointed this out to us. We removed those pillows, opened the the overhead vents a little, problem gone. Our Class C is 25 ft long and I was told thet even two people breathing can cause some condensation at night.