What year, make, and model is the camper ?
If the cabover bed area is wet, the mattress will usually be wet too.
First thing would be to get the wet mattress out (if that's what wet?), and bring it inside to dry out.
Next best thing would be to pop the camper roof up and run a fan, or a space heater and a fan inside to get it dried out. Let it run all day and all night if you can.
This will really help to dry things out like the folding lift panels, headliners, insulation, walls, wood, etc.
Then you might want to check the roof and make sure the screws are sealed up and tight, check the seals around the vent, and check for any holes in the camper roof. I was helping a friend with his older FWC the other day and ran into that. His camper is approx. 20 years old, the roof screws were all tight, and the entire roof was coated in some sort of RV roof coating. It looked good and I didn't see any issues. The next time it rained, there was water on the cabover bed mattress? I got a small ladder and looked at the roof again closer, and sure enough there was a small puncture about the size of a dime. Helped him patch it and seal it up, and he's good to go now. Start with the simple & easy things first and get the water out to stop the mold and musty smell.
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