Yup, he is "trailer sour".
Is this the same horse you are wanting to use a tie down on?
You need to MAKE him do what you ask him and make his listen to your leg. No argument. He should not be ignoring you. Same as how when he is "spazzing" a show, he should still be listening to you 100%. No argument.
You don't need to beat the snot out of him to get your point across, but you do need to be consistent, persistent, and firm. If he ignores your leg and makes his way to the trailer, make your leg cue stronger, and do NOT stop cueing him until he moves away from your leg pressure, as you are asking. I don't care if you have to cue him for 5 minutes before he listens. Just do not give up and do not stop asking him without a correct response. If you do, he's won and he's learned how to press your buttons.
Another help can be to work him right by the trailer (if space and footing allows). Trot and gallop circles next to the trailer which makes it a place of work. Then go back over toward the arena and allow him to rest, relax, and stand.
Either way, whatever problem you are having with him (him not listening to you), you've got to be the alpha leader in your little herd of two, and do NOT stop cueing him to do what you want until he does it. You need to "win" every single time. Or else he will learn to walk all over you, as he is doing now.
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.