So when you switch your AC on, the engine cooling fan on the radiator should come on to help airflow through the radiator and AC condenser when the vehicle is not moving. If it does not come on, then the lack of airflow through the condenser will hurt the performance of the AC, making the air inside the truck warmer and closer to outside temp. Can you let the vehicle idle, turn the AC on and see if the fan is spinning? Possibly turn it on and off and see if the fan responds.