My iPad camera roll says it is using 7.5 gb after all pics have been deleted for weeks
Plug your iPad into iTunes and verify that there are no folders of photos from your computer that are being synced onto your iPad, it is possible that although you have deleted all of your photos that you took on your phone (i.e. the ones in the camera roll) you could still be syncing a folder of photos from your computer.

This link from the Apple website should have all the information you need about how to check that if you are unaware.