My rescue pony *hated* the trailer when we got him. So my trainer worked with him for a month. During the day he was in his stall, but at night and feedings he was in a round pen with the trailer backed up to the round pen. She started with the bucket at the back of the trailer, and every few days she would move the bucket a few steps into the trailer. We did this for a month, and it worked. Now I have to keep the trailer door closed, other wise he just stands in it.
When I got my newest horses, neither of them really like the small straight load play day we had t the barn. So I did the same with them over a week. (Didn't really *need* them in it, just so they wouldn't freak.) They were willing to walk into it, but didn't want to stay.
~*~Never approach a bull from the front, a horse from the rear or a fool from any direction. - Cowboy saying~*~