Clear up space on my phone that seems be taken up by recently deleted photos?

I had over 4 GB worth of photos on my phone and a month ago I deleted them all.

As of recently, the 1600 or so photos in the "recently deleted" album were automatically deleted. The usage section in the Settings app says that I still have over 1800 photos and that I only have 1.5 GB of space left on the device.

As far as I can tell I have less than 100 photos left in the Photos app.

I've rebooted my phone and even hooked it up to iTunes but nothing helps. Any way to fix this other than completely erasing everything and going back to factory settings?