“I’m protected under the ADA. This is my service dog. You may ask only whether the dog is required because of a disability and what work or task the dog has been trained to perform.”
HIPAA applies to covered health care entities and related business associates, not to ordinary stores, restaurants, truck stops, or most other businesses you enter.
That means people often say “HIPAA” in access disputes, but the more accurate point is this: you are generally not required under the ADA to show medical documentation to a business to enter with a service dog.