While some regional or department-specific slang might exist, generally, officers would simply refer to it as a "morgue van," "body wagon," or "meat wagon."
It's important to note that these terms can be considered insensitive and disrespectful, especially when speaking with the families of the deceased.
Instead of relying on slang, it's always best to use respectful and professional language when discussing sensitive topics like death and transportation of the deceased.