This is actually the most frequent problem associated with RV air conditioners, so don't panic. When it happens, you’ll have to monitor all the parts of your AC to detect where the leak is coming from.   Usually, there's an issue with the rubber gasket, so check for the monitor bolts and tighten the gasket to the roof. Also, if the gasket has worn out or become brittle, getting it replaced immediately is always a good answer.  Occasionally, leaks result from damage to the roof and not the AC unit. Inspect your roofing to check for any kinds of leaks, sagging, or deterioration, and you might have to install another gasket to prevent water from pooling up.