Did the driver actually open the trailer at any point on the trip, like at a weigh station or a fuel stop? Because a sealed trailer should not let in that much dust. The vents are usually filtered, and most of the dust I've seen in covered shipping comes from the car sitting outside at the pickup or drop-off spot, not from the road. If your car sat in a Phoenix lot for a day before they loaded it, that fine desert dust would settle on it no matter what kind of trailer it rode in. Check your bill of lading for when they picked it up versus when it actually got loaded. That gap is usually where the dirt comes from.