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.
We must be willing to get rid of the life we've planned,
so as to have the life that is waiting for us.