Q: What will happen to the photos on my iPhone 5 if I turn off Camera Roll under Manage Storage/This Phone?

They will not be erased from your phone, they will only not be included in the iCould backup anymore.

If you take many photos and videos with your iPhone, iPad, or iPod touch, you might notice that they take up most of your iCloud storage. To check the size of the backup on your device in iOS 8 or later, tap Settings > iCloud > Storage > Manage Storage. (If you're using an earlier version of iOS, tap Settings > iCloud > Storage & Backup > Manage Storage.) Then tap the name of your iOS device. The backup size is under Photo Library.

To reduce the size of your Photo Library backup, save your photos and videos to your computer, then manually back up your iOS device. If you want to keep your photos and videos on your iOS device, you can turn off Photo Library in Backup using the steps above or purchase more storage. If there are any photos that you don't want to keep, you can delete them. Just open photos and select the item you want to delete. Then click the Trash icon and tap Delete Photo.

copied from:

iCloud: Managing your iCloud storage

Look also in any of the messaging apps, cause they can store huge caches of old messages, some of which may contain pictures, animations, etc. Whatsapp, Telegram, Messages, Zello, Viber, etc., are the usual suspects in this case.