Yes, cardboard of most varieties and or many fabrics hold moisture (water) for a very long time and if there's not adequate moving air ventilation, mold will grow. Almost guaranteed! The higher the humidity and ambient heat level, the faster the mold grows also.
The OP lives in Michigan and so do we but on the west side of the state where the ground is mostly a well drained sand base and sand dunes. If the OP lives in an area with lots of clay content in the soil, the humidity level close to the ground will be very high especially during periods having weekly rain a day or so and then hot sun evaporating the moisture up out of the clay.
We store our RV's always with 2 to 3 windows on opposing sides open about an inch or more and also have one or more roof vents open because we have Max Air roof vent covers on all our 14 X 14 roof vents so rain can't come inside. Also, all of our RV's have crank out lower window bottoms (we order our new RV's with them always) and of course when open an inch or two, no rain ever comes in. We're on our 51st year of RV'ing and have learned what works really well. Sometimes when there's been a lot of rain (doesn't happen very often) I will put 1 or 2 Damp Rid containers in the 5th wheel basement if we aren't going to use that RV for a couple months. Consequently, due to these various actions taken, we have never had any mold grow but our 3 homes away neighbor who take no preventative actions did have mold in his Cougar 5th wheel along the bottom of some of his walls. He killed it with Lysol spray liberally applied and then left his one covered roof vent open and 2 of his horizontal sliding windows open a couple inches with having the purchased sliding window plastic vents he attached to the outside of the window frame. He told me he emptied 4 cans of Lysol spray as he did all the lower walls, inside all cabinets, basement storage areas, all around the toilet area, etc.
The too he may have a leak but he's pointed out two different areas with mold so unlikely to be a leak.