Mine did that. Taking the AC out is very straight forward (just remove the screws on the surround and slide it out.) After that, you can see how it's mounted. If it's like mine, it'll be sitting in an aluminum tray that has been hand-bent to hold it. Look at the back of the tray to make sure it provides some way for water to drain outside of the wall. Mine was mounted too far forward. I ended up sliding it back, fastening it in better, and drilled a weep hole in the corner of the plastic outside surround for the water to drain out of.
While you're at it, look at where the side air intakes are on the AC unit. These are the intakes that are used to remove heat. Make sure the aluminum doesn't end up blocking them off. For the unit to work well, it needs to have good air flow. Mine had a side intake on the right side, and the aluminum nearly completely blocked it off. I cut the aluminum so that the AC could breath properly. It pulls air in through the roof vent, sucks it into the sides of the unit, then pushes it out the back through the cover on the side of the trailer.
Since doing these things mine has worked very well indeed. It drains to the outside, and is able to keep the trailer cool easily. I even used it in August in Florida, and it easily kept the trailer comfortable.
2014 Innsbruck Lite 198BH