Had the same issue in our 19H. Its condensation. I have researched it to no end. I purchased a vinyl mattress cover from Bed Bath & Beyond for $15. Covers the mattress like a pillow case. It has helped, but I have still noticed a small amount between the vinyl cover and the mattress when the temp outside is cool at night. The next thing I will try is the 1" thick foam insulation board from Lowes. Cut it to size and put it under the mattress.
A buddy of mine has a Roo hybrid and swears he has never had this issue. I wonder if their bunk end doors are better insulated?
2005 Dodge Ram 1500 Quad Cab 5.7 HEMI
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