When I still lived up North I had same problem.
What I found was with the cold temps and then warm up condensation sometimes builds up under mattress where it touches the wood base. Because each is a different temperature when the temps outside are warming up.
It doesn't really mold up it just gets damp which gives you that mildew.
I have allergies so the first time that happened I had to throw out the mattress.
What I did was put four big empty 5 gal paint buckets on each corner and lifted the mattress on top of them for when MH was in storage for the winter. No more moisture problems.