Along with what the others have suggested, work him outside the trailer...alot...do lungeing, sending exercises, backing, etc...everything BUT going up to the trailer, but being near it. Stop him for a bit, and send him toward the trailer, to investigate...when he takes a few steps toward it, then take him away from the trailer. Resume some ground work, and return to the trailer, and do some sending exercises infront of the open trailer doors. Back and forth...when he is calm about going between you and the trailer, stop him and ask him to once again face the trailer, and sniff, or set a foot in...then walk away, and resume some work. Gradually asking him to set two feet in before backing him out and going away from it...eventually he will go in all the way, and again, you will back him out before he can panic, and do some more work. Gradually let him stand in the trailer longer and longer...then taking him out and resuming work. Teaching him this way teaches him that the trailer = his only rest, so it turns out to be a good thing, and almost "his" idea to want to be there.
To teach him to become used to the slant dividers, get him to go in, and just move the divider back and forth, but not really near him. Do this maybe once or twice the first time you start, and then back him out again. gradually swing the door more, and more, getting closer, and closer, and rewarding him by backing him out WHILE he's calm...You have to really pay attention to his body language; you have to move him out before he tenses up and starts getting fidgity, so if you can only grab the gate the first couple times, fine...that's where you start. Work with his comfort level on the divider part. But since you will have worked with him to make loading his idea, he will likely be alot calmer by that point.
This will take more than a few sessions to sort through, unless he is one of those really smart horses who catches on quickly. Take your time, and do it right
I would get him loading well in one session, and the next do some loading, and desensitization with the gate, etc.