Why was my late father's ambulance transfer denied?

He was transferred from Assisted Living to a nearby Hospital for kidney failure (when a catheter was put in, he improved and was discharged several days later.) He was Hardly Healthy, However. He passed away one month after discharge at Assisted Living, under Hospice Care. He was Ninety years Old. Medicare felt he didn't need an ambulance, or wouldn't pay the deductible. The Bill is over four hundred dollars. Wisconsin Physician Insurers (who seem to be in charge of approvals/denials) are stonewalling me because I didn't prove I was the Executor of the Estate. But I sent the Death Certificate, and transfer of Deed for my inherited house. They also say in their Denial letter that I have NO more appeals Left.