Which is work him outside the trailer and let him rest inside.
I don't really have access to a trailer all the time to practice this method with him. But I wish I could.
If not what is your method?!
I have a horse who will plant his feet and just not go, he's not scared just stubborn, so we've had to use whips, not hitting him hard just tapping him on his butt until he gets annoyed enough and walks on (which can take a few minutes).