The hidden sin of your ipad is the stored cache and garbage collection. These are not shown in your usage breakdown. The bad part of these is there sometimes are no means of getting rid of them short of copying off all the data and re-formatting the ipad, then putting the data back.
I had a 16gb ipad at last year. (i since sold it for a 64gb version) and ended up at one point with 7gb of cache and garbage collection. DON"T LISTEN TO APPLE when they say that plugging into iTunes and doing a backup will get everything. From my experience it does not. Backup all your photo's yourself. Backup everything that you can't afford to loose. Then you can factory reset the machine, and restore from your iTunes backup, and recover the lost files from your own backups.