The rooftop AC is a sealed system and in general does not have any leakage, even after 15 years. It is not like an open system in a car or truck which has rotating seals and rubber hoses connecting things that leak over time.
The auto AC units have a low pressure switch on them which may allow the compressor to run momentarily until the switch shuts off, indicating lack of refrigerant. The sealed rooftop AC units have no such switch.
After 15 years of no use, the first thing I would consider checking is the start and run capacitors, and the relays associated with them, not the refrigerant level.
Rooftop AC units and automotive AC units have similar parts such as fans, compressors and condenser and evaporator coils, but operate and are serviced differently.