Here in Louisiana where humidity levels are quite high, I use an electric dehumidifier to keep things fresh.
Before I used it, the walls were sticky and it became quite musty and unpleasant. With the dehumidifier removing a surprising amount of water, it always feels dry and even though it still gets hot, the trailer stays in much better shape.
I have my trailer under a roof. It it's not, the trailer should be inspected periodically for any leaks after rainfalls.