Coleman rooftop AC not cooling - UPDATE

Thank you for the replies. The consensus of a leak was confirmed. I installed a saddle valve to test for pressure and the system was virtually empty. I pumped nitrogen in and found the leak right at the connection of the suction line into the compressor. I plan to repair the leak, remove the saddle valve and braze in a service port, evacuate and recharge in 18oz. of R22. I still need a service port(cheap), micron gauge(don't have one, can't get one, will just have to wing it), and a scale(borrow).

It's a sealed system like a fridge or window A/C. Probably a leak and possibly where you haven't looked or can't see, like the evaporator inside that metal housing in the front end of the unit. Chris Bryant offered an interesting test: Run the thing a few minutes then turn OFF and immediately back ON. If it's full of refrigerant, the compressor won't be able to start and it'll hum then shut off on overheat sensor. If it starts right up, it's empty. Another OP, Mr. Landry, mentions that there's a coupling between motor section and compressor section that can fail. Then a charged system will restart but not cool since the compressor's disconnected from the motor. But either way (leaking system or failed compressor) the best fix is usually to replace the whole RTAC (Roof Top Air Conditioner).
Just be sure the compressor is starting and staying running. If it isn't, could be a failed Capacitor or Hard Start Device.

Refrigerant leak. AFAIK, and I could be wrong, they are not like vehicle HVAC systems where coolant can be easily added, so there is a good chance that the whole roof A/C might have to get replaced.

Maybe someone has a better idea, or a better fix.

The ONLY sure test is an AMP draw reading of the compressor. Doug

Classic low refrigerant symptom.