I found with my 77 Sovereign
, that the tail lights were allowing so much water in, that they were rotting the floor in the back corners. I replaced the lenses, made gaskets and caulked it from the inside as well as the outside. That stopped the leaks.
I also had leaks coming in from the lower trim, but after I replaced the rear floor, I put aluminum flashing under the trailer skin to deflect water away from the flooring. It has been over 5 years, and no more water coming in at the rear of my trailer.
I hope that helps. Check your tail lights.