Good luck at Mayo...

Not many of our PNers have much good to say about them.
Mayo is very conservative and mostly anti-supplement OTC etc.

They will test you alot and tell you what the results show hopefully. I'd suggest getting copies of everything. One poster here in the past had POTS and was not told, by Mayo after her testing. But when she requested copies of all her testing, she found that diagnosis in them.

Their B12 ranges in their B12 testing are still the old outdated
format. So get that number specifically.(you need to be above 400pg/ml) An MMA test to go with it would be helpful too since you are getting many tests.
Get Vit D tested too. The RX version to treat low D is not effective in humans and doctors still prescribe it. So bring your numbers here and I will give you a suggestion for D3 OTC alternatives.

Make sure they test you for amyloidosis...and full blood work for immune globulins etc.

Since you are fasting, and dieting strenuously, it would be a good idea to have vitamin levels measured, and also all minerals/electrolytes.

I don't think it is wise to fast for days, as you have said here while using oral antidiabetic medications and insulin. You should ask your doctor to advise you on a minimum caloric intake for your medications to work properly. Otherwise you risk low blood sugars, and lows can damage nerves.

Don't take any vitamin or mineral product before testing. Best to be several days off of them if you use any.

Good luck at Mayo...I hope you find some answers there to help you.