We have a 39mb and had the same musty smell in the locations you mentioned.
I agree with earlier reply, make sure ac unit & any filters in the system are clean.
The area’s you mentioned can can have moisture build up from cooking with both stove and or microwave. Use ceiling fan to vent when needed.
We found, it’s all about the % of humidity in our trailer. We keep out humidity around 40% and no more musty smell.
When ac’s are running a lot that keeps the humidity throughout the trailer in good shape.
When ac’s are not in use, we have a small dehumidifier we run & set at 40%.
We are from Michigan and use our 5th wheel to escape to the south for the winter months. In the sumner it is stored for long periods so we run the dehumidifier during that time as well…��