air conditioner drip

I have just purchased a 2007 Spree. So far we have loved it and everything seems to be working just fine, including the air conditioner. The only problem is, it drips. While running it drips water from the return air vent. This is condensation that is not draining off. I was wondering if anyone else has had this problem and what a solution might be. I am sure that the trailer has not been used for at least one season and feel sure that the drain is plugged up and was wondering how I might go about clearing it.
Thanks for any help with this;