Maybe some of these other possible reasons for the OP's blowouts are correct, but the first thing I would check are the tire valve stems. The valve stems, especially if the tires have been on the trailer for a while sitting in the sun and weather, may crack and become brittle. You can have new valve stems put on your tires for a relatively small price.
I post my reply because I experienced this problem on my trailer when it was only 2 years old. I had just purchased my 5th wheel from the original owners who had kept it down in Florida for two seasons before bringing it back to Michigan to sell. The tires still had a lot of tread on them, but the weather had taken it's toll upon the valve stems. Always check those valve stems!