It's not Freon but 134a you can but a recharge kit at any auto parts store. Usually you buy the first can with the charger head for a little more money then additional cans if need are less. I've done it to mine and 2 years later it's still good. A friend has also done his with good results. Class A with diesels take a little more 134a as the compressor is in the back on the engine and all the piping to get it to the front. Usually there is gauge on the charging head to tell you when you are full. Just make sure you get the charging head that takes the screws on bottles.
Try it what have you got to loose if it doesn't work you will have to take to a A/C repair anyway.