RAID Disk Space not Recovering
I had the same situation two days ago, not on a RAID although I doubt that matters, and I ran Verify Disk in Disk Utility on the drive and it said "Volume bitmap needs minor repair for orphaned blocks" so I then ran Repair Disk and it cleaned it up and my free space was what it should have been after emptying the Trash and it wasn't at the time.


Note: I ran Verify Disk and Repair Disk in Disk Utility and was backed up before doing so.  As a general rule one should always have everything continually backed up and especially before performing Disk Level Operations of this nature, even thought I've never yet had a disk messed up doing so.