I also have black streaks running down my rear picture window area on my 5th wheel trailer... I think one could solve the problem by placing a long 1/2-inch angle strip across the roof near the rear edge. You can use Dicor Sealant (No screws) to hold the aluminum angle strip in place. This would route the run-off water to each side of the roof to run off using those add-on spouts you are referring too.
I purchased the aluminum angle strips but never got around to installing it... haha
I got the idea from my OFF-ROAD POPUP trailer roof area which already has this J-strip rain gutter installed like shown in this GOOGLE on-line photo.

Roy Ken