Our bank has always reported personal guarantors of a corporate loan under the guarantor's personal credit record. Now, we've had a couple of customers complain that they went to purchase a new vehicle, and it shows that they have $3 million in loans. Where can I check to see if we are required to report this way or not?

You are not required to report this way. These are contingent liabilities, not direct liabilities, and generally only are carried on a personal financial statement, not on a credit bureau report. I have never worked at a bank that reported the guarantors of its commercial loans to bureaus (I have worked at banks with assets from $2Bn to the very largest.)

Is this just a generally accepted bank practice or does Reg B or V cover this?

I don't believe I have ever seen hard and fast rules on this. Reporting to bureaus is not a required/regulated issue except that what is reported is correct.

Often contingent liabilities such as this are not reported as often commercial debt is not reported. If you saw it on a report, it should prompt questions to the applicant. This is not indifferent to a doctor's financial statement. There is usually a question about being subject to a suit and the answer is often "yes" because doctors are sued all the time and they can go on for a very long time. The question prompts other questions and shouldn't be a deal breaker.