My mare, Athena, was terrible about trailers. She still doesn't like to walk by them, and will side-pass when we have to go by one so she can get a good view of it. Lol
It used to take up to 3 hours (!!!) to get her in the trailer, until I finally found a trainer that had a technique that worked.
She had me walk straight up to the trailer, confident, acting like she was going jump right in there. If she did, great. But mostly she would stop right at the edge of the trailer and stretch her neck as far as she could. This was when the trainer (or who ever is with you) took a lunge whip and tapped her hind legs until she took a step forward, and then stop tapping when she did. (So simple, why didn't I think of something like that!) Sometimes Athena would have a hissy fit, do a little half rear, and try to back up. This was when the trainer had me back her up, walk her around the trailer, and try again. Now I know, I didn't think it was a good idea to take her away from the trailer after her hissy fit, (Idk, maybe, since she is a girl, she needed a few seconds to cool off. :P) but after doing this whole thing a few times she has learned to walk right in on her own. So I'm not complaining.
I think the whole 'thinking the trailer will attack at random when you walk by it' thing is just a little trust issue that I still have to work with so she realizes that trailer's aren't deadly.