Most rental companies specifically disallow towing anything other than towing equipment they have rented to you.
Rental car companies renting trucks will generally put a locking pin in the receiver to prevent you from connecting to it.
One certain rental truck company has a specific, proprietary size hitch ball installed on their trucks that only works with their own towing equipment so that you cannot connect a standard size coupler to it.
The bottom line is that rental companies, and specifically their insurance companies, do not want you towing equipment with their vehicles.