Hopefully you'll have posts available... But if not, it'd be good for him to become accustomed. I'm nervous leaving them tied to trailers, there's too many edges and things for them to get snagged on. If he's the only horse with you, what about leaving him loose inside the trailer? Tiff and I did that with her big stock trailer, one in each compartment. Was comfortable for them, and safe.
Are you intending to practice tie him to it with the truck still attached? Or is it heavy enough you know he can't move it if he got into a pull back situation?
Tieing to your trailer is required at a competition. Every horse has to be confined the same way so no horse has an advantage.
I'll block the tires so he can't pull it.
Becky