If it were me, I would fix basic holes in training, and halter pulling is a big one for me.
If you have someone to show you how to apply a body rope correctly, I would get that help. A horse that halter pulls, doe snot lead with respect, anywhere you ask him to, is also not solid on giving to pressure.
Now, in all fairness, once you have that, you have to amke sure that there is no reason a horse fears being in that trailer, so, do you have dividers, that do not allow a horse to spread his feet, to balance, when moving?. What about head space? I have seen some 16.3hh plus horses, forced into trailers where they have to travel with their heads lowered
Yes, you should most likely put up that butt rope, before tying , which might make many horses think twice about setting back, but , which also can cause a wreak with a horse that really sits back anyways. Bandaids are never as good as fixing the real problem!