x2 what Chris said. If it was cold and simply stopped, it could be as simple and inexpensive as a connector/connection, capacitor, switch etc. We had a Coleman that age that was blowing cool not cold and cleaned it per instructions Chris has on the RV.net Blog. It cooled very well again after that.
Your problem sounds like the Fan runs but the Compressor does not. Blowing but not Blowing COLD, correct? From my experience, I would not condemn a rooftop A/C on age only. Now, if diagnosis indicates leaks or a bad compressor, YES I would not fool with it. Would replace.