The unit also has a freeze sensor, so I am wondering if that is working properly. If you look at my signature, there is a link to a service manual -- under Coleman and then 1976376.pdf. It will walk you through some of the troubleshooting steps. If this does not match your unit, you should be able to google search and find the right one.
These AC units somewhat put you in a bad spot. They are like $600-$700 new plus install if you don't do it yourself. You are going to spend several hundred on having it worked on - so are you better off bitting the bullet and just replacing it or having the old one repaired? The system is sealed, so adding additional freon requires them to add ports - not sure how expensive that is - plus repairing the leak. Tough call.