Finally we are working on going into the trailer and having the door shut and waiting quietly for a few minutes. The first time I did this he was very good inside the trailer but then I had to push him off the trailer because he was convinced he was stuck inside forever and ever. After that we had to go over him saying NO! again because then he didn't want to get back on.
I worked some more on loading and staying on when the door was closed and he proved to be a quick study. He knew when the door was closed that he couldn't get out and I reinforced this by saying "whoa." Then when the door opened I had him wait and then said "back" several times so he knew he could only get off the trailer on a verbal OK. He's confident enough in the trailer to look for things to eat and stick his head out the window to see what's going on.
When he does all of these things instantly and consistently we will move the trailer to a different location and see if he still loads, then I need to decide how I want to haul him before we go for a ride. I can haul him loose, or teach him to stand for the divider to be closed and I need to decide if I want him to be tied or not. I know that people have gone on trips with far less work but I want to do it right, I don't want to get stuck some place because he doesn't want to get back on the trailer, he needed the groundwork to help with his RESPECT issues, and I want the trailer to be a positive experience and not just a thing that takes him away from his life.