marpel,
I had leakage like yours inside of one of my trailer's pass-through storage doors.
My storage door frames were secured to the trailer with screws through the metal frame, outside the gasket, around the perimeter. Rainwater was pooling along the bottom of the frame, outside and below the rubber gasket.
It occurred to me that the ONLY way for water to seep into the storage area was through the one or two screw holes that were securing the bottom of the frame, where the water was pooling.
After drying up all the water, I backed-out the lower frame attaching screws about 1/4 inch. I put calking around each screw, under the head, and screwed it back down. That stopped any more water from seeping inside the compartment. Hope this works for you, too.
Ed