Phone memory shows as nearly full in PC, but is not ATT version of Galaxy S3
running cm-10.1.2-d2att.zip 

http://download.cyanogenmod.org/?type=stable&amp;device=d2att

I seem to be having a storage issue. I'm guessing it was while I was changing to a new a ROM. I wiped the device and did a fresh install, but used titanium to restore settings. 

the 12 gig internal reports having less than 1 gig free, but I can only find ~3 gig of information. (see pic)
Try reflashing the Rom with full wipe again but don't restore anything with Titanium Backup.

similar problems usually happen when I used to restore data with Titanium Backup especially if the android upgrade from Rom to another is significant (e.g 4.1 to 4.2). 

please try this and tell what happens :)

Edit: added my comment to the answer.