Kudos for taking in a rescue, Nu! I've seen far too many race horses beat into trailers with brooms, because the trainers/owners only cared about conditioning to run.
When I've been in your shoes, I'd take a break from even looking at a trailer. I'd focus on perfecting our leading/following on loose lead every where I could possibly go - up & down hills, left & right, serpentining like a drunken sailer, walking over bridges/plywood/poles/empty feed bags/floor mats, walking in and out of the barn. A few weeks later, I'd walk purposely into the trailer expecting my horse to follow as they've done every where else. No guarantees, but at least they've had a break from the "work" and/or anxiety of being asked to go into a trailer regularly and they are more likely to follow me in.