In the meantime, I'm having similar difficulty (even with a copy of the death certificate) convincing the California Department of Motor Vehicles that my mother's car (which was accidentally totaled shortly after her death while it was being moved, but that we haven't been able to actually dispose of due to the cloudy title) should be in "Planned Non Operational" status and that I have the right to do so. This being even after I sent them a copy of her death certificate, will (naming me as executor and sole heir), and the necessary fee to put it into PNO status. They cashed the check, but they continue to send her (at her old address, not mine) dunning notices for registration fees.
You'd think they want the people who die to come in to their office before they die to tell the people there that they'll be dying at a future certain date and would they please update their databases.