Coleman rooftop AC not cooling - UPDATE

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.