It could be due to a plugged drain off the drain pan, or your Hi-Lo being off-level. Check the level. If not that, send somebody up on the roof, have them remove the a/c shroud and look at the drain off the drain pan. On my 2008, the drain isn't a tube, but a small depressed area coming off the passenger side of the RV. Mud daubers sometimes plug up the drain with their nests.
If neither of these, it could be dirty evaporator coils on the a/c. If that's the culprit, an air compressor or vacuum can be used to clean the coils, also accessible by removing the shroud.
Finally, it cold be a bad gasket that seals the a/c unit into the roof.
Good luck, let us know what you find.