I was having the same issue. Under the "Usage" section in options it was showing that I had over 5gb of information in Photos & Camera. The "About" section showed over 770 photos and 99 videos. I had deleted every photo and video from my camera app, including the recently deleted photos, and all photos under other albums like snapseed etc. I also deleted all of my text messages just to be sure it wasn't seeing the photos there. The problem persisted.
I tried various methods of syncing through iTunes, including syncing my photos to an empty folder. Still, no solution!
What eventually worked is I opened he photos app on my iPhone and deleted every folder, instagram, snapseed, go pro, etc. even though they all showed "0" photos I found that my disk space freed up. I still have 26 unaccounted for photos, but I freed up the vast majority of wasted space.
I hope this works for some of you others having the same issue.