That water should not get that far back. Usually the bumper trim covers the lap seams from the upper and lower skins. Sometimes the seam from the lower panel ends up outside the upper panel which if you have a leak in the top of the bumper trim you could have problems. I sealed this lap seam with Trempro-635 before installing the trim.
This thread might help you. This the way I did it. I plan on putting the bumper compartment back on but I will need to extend the bumper a few inches to I have a gap at the back for water to shed. I will make a self contained box that is separate from the belly skin of the trailer.
I have thought about drilling some holes in that rear bumper plate to drain the much of the water away, so it does not pool back there. I don't want it to come from underneath either when towing in rain.
They added a factory seal between the shell and the frame on the new ones, but I still do not like the gathering of water there.
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