I assume your trailer leaked when it was stationary. If not, your leak could be on the roof or on the bottom of the trailer where water washed up from puddles in the road.
Any place where there is a hole in the roof for a vent, t.v., antenna, ect is a place to look for leaks. On an older trailer, particularly one that has been damaged, the leak could be coming from an open seam where one sheet of aluminum joins another.
Your leak sounds pretty big so it should be easy to find.
My Safari leaked around the refrigerator vent. I had to caulk it twice before it stopped leaking.