Just a thought ... If you're in a hot & humid locale (like the upper gulf coast) you can easily ice up your unit and it'll just blow warm air until the ice melts. I've had that happen a couple times while down at the beach in Galveston this time of the year.
It's broken. OK I could not resist. Low freon, compressor problems, electrical problems, thermostat... There are a number of web sites that can help with diagnosis. One such http://www.money-saving-rv-repair.com/rv-air-conditioner-repair-and-troubleshooting.html I do however disagree with one statement the systems can be recharged with the addition of fittings to the lines.