This often happens with mattresses with fabric bottoms.
It's from warm bodies and colder bunk doors, which cause condensation under the mattress.
Some have cured it with solar blankets.
Some with sheets of foam insulation.
I use a wool blanket that allows air to pass through.
Dan-Retired California Firefighter/EMT
and Zoe the Wonder Dog(R.I.P.)
2016 PrimeTime TracerAIR 255, pushing a 2014 Ford F150 SCREW XTR 4x4 3.5 Ecoboost w/Max Tow Package