If you have a drip pan, then the drip pan will probably have been sealed to the roof with some kind of caulk, and then the air conditioner sits down inside the pan. The AC seals to the pan with a thick foam gasket. The sides of the pan are sometimes stuck to the AC by means of some kind of double stick tape, and sometimes not (so I have read). It is conceivable that in a hard rain, water could run down the side of the AC, and get inside the drip pan.
The pan should have small holes at the bottom of each of the long sides that will allow rain water to drain. It could be that these holes have become clogged and are not allowing the rain water to drain, thus it is getting into the condensate drain area. If the rain water gets into the condensate drain area, then it will try to go down the condensate tube just like condensate. If your condensate tube is running freely, you should never notice any leak. It sounds like your tube had some blockage.
So, things to look for:
-Check around the entire perimeter of the drip pan for damage (cracks, breaks, etc.)
-look for the drain holes in the long sides of the pan and make sure they are not clogged
-ensure that your condensate tube is completely clear
-it is possible that the foam gasket that seals the AC to the drip pan is shot, allowing water to get into your trailer from around the AC. You might have to remove the interior ceiling airbox in order to see the foam gasket and look for leaks in it.