AC works well in city but after 2-3 hours on highway it blows warm air

Check for any possible restriction at the radiator and condenser -- like debris around or between them blocking air flow. Check for compressor coil and clutch operation. If the system has sufficient refrigerant, check the electrical operation of the system once you confirm there is no debris. I would mention condenser fan as a concern but you say it blows cold around town...so...