Good suggestions here... here is another one...
Make sure that the trailer seems "open" enough... (ie: Take out dividers, open windows, etc...) to make sure there is enough light in the trailer.. take the trailer to an area where it can be backed up into something that will make the trailer floor, (if you do not have a ramp) level with the ground. Take a large SOFT rope and make a butt rope out of it, with the end threaded through the bottom ring of her halter. Have someone thread the lead through the front window of the trailer and, wearing gloves, start a slow pull backwards on the rope to snug it up on her hindend. At 900lbs you should be able to have two men lock arms behind her and while the rope is being tightened behind her, start pushing her forward towards the trailer. Above all, make sure the handlers are staying CALM and praising her everytime she makes a step forward. I don't know that I would use Rompun as I have seen alot of horses that just do not react well to it. Talk to the vet about an alternative..
Best of luck with her...
Suzy Hooper
Show Horses by Suzy
Fresno, CA