I feel your pain with this one! When we went to rescue AZ, he was checking out the trailer, smelling it, etc. which was taking some time but it was a new-to-him trailer. I was patient.
The former owner was not. He came out with a freakin' whip and whipped the snot out of him till he loaded. I was SO MAD... but I just wanted to get AZ out of there before the man got uppity and demanded money for him or something. He is a shady person. gaah I hate that guy.
ANYway. Patience. Patience, patience, patience. Do as others have said. Start with associating the trailer with food. Walk her up to it. If she shows the slightest bit of interest, praise her and give her treats like she just won you the Kentucky Derby. Let her take her time. Make sure any time you are near the trailer, all attention is positive and reinforcing. That will take the "omgbigscaryboxthing" fear away.
After that is accomplished, do as the others said and associate resting with the trailer.