It was conventional wisdom for doctors to say that opiates don't help nerve pain, but recent studies have contradicted that. Many people with PN pain are on a combination of gabapentin or Lyrica, an opiate, and possibly an anti-depressant.
If you have severe pain, not just the burning, zapping kind, I don't think maxing out your gabapentin will help. For myself, it only seems to help with the burning, zaps, and hamsters running all over my feet and legs. It does not remove those sensations entirely, nor does it touch the crushing, twisting type of pain. Was the cocodamol helping?