i am one of those people who pass trailers. also dark window cars, vans, trucks, buses etc. it is not that you are going too slowly, it is only because I CANNOT SEE AROUND YOU.
don't get mad, when you are towing your trailer, let people pass you safely.
at one time i was the safety coordinator for a six state region of the largest company in the world. we were trained to look ahead and behind at all times so we could take action if needed to remove ourselves from danger. that includes being able to see that the car in front of the car in front of the car in front of you is going to turn left and one of the cars in that train doesn't look as though it sees that action. so.....if i can't see, i will pass.
please don't hate me or anyone else for this.