Your doctor should know that neuropathy is one of the long-term complications of diabetes as a result of high blood glucose levels damaging the small blood vessels which supply the nerves. This prevents essential nutrients reaching the nerves which then get damaged or disappear.
A B12 deficiency can be confirmed with a blood test but the blood serum measures ALL the B12 in your blood and there is only a small proportion that is "active" and works within the cells. However, there is a test measuring only the active B12 in your blood, detailed in following link.
The test is also available on the national health at St Thomas' Hospital in London and apparently you can send a sample with a GP's letter but I used Blue Horizons private testing and it cost about £70.00. I have neurological problems in my extremities too but my active B12 is high so this was not my answer.
Neither my GP and haematologist had not heard of this test so if you plan to approach your doctor, it would be best to go armed with info on it.
Hope you find your answer soon,