I also experienced great difficulty in obtaining insurance today for a 1979 Ambulance. USAA, Progressive, and a commercial insurance company refused to insure the ambulance. After I provided USAA and Progressive the VIN I was told it was a commercial vehicle and they would not insure it for personal use. I told them it was decommissioned (i.e. no lights or siren) and the Title shows Ford Econoline 350.
USAA transferred to a commercial vehicle insurance company to obtain insurance and they told me I couldn't get commercial insurance because I was using the vehicle for personal use.
I then called an insurance broker and explained my issue with USAA and Progressive, the broker tried multiple other companies that denied insuring the decommissioned ambulance. Then Success Happened. The broker then was successful insuring the ambulance with Hagerty Insurance. I was required to provide exterior and interior photos and insure the vehicle off of stated value. Not the cheapest insurance, but it is much cheaper than commercial vehicle insurance.