Once you delete the photos and videos, double-click the Home key and close the Photos app by swiping up. Then check your storage again. Also, in the Recently Deleted folder, have you done the following to make sure you don't have residual photos that were left back in iOS 7 due to an issue with the aging off of deleted photos:
1. Go to Settings>General>Date & Time, turn off Set Automatically, and set the date to 09/01/2014
2. Go back to Photos and open the Recently Deleted folder to see if you have residual photos and manually delete them
3. Go back to Settings>General>Date & Time and turn Set Automatically back on
Cheers,
GB