We put on an alluminum RV gutter simular to the ones you see over slide outs on many 5th wheels and trailers, Also used gutter ends and extensions. Put it just above the strip in the back. I slanted it towards one side about an inch higher on the right. Use putty tape uder the alluminum before you screw it on and a good calk along the top edge.
Terry & Sharon
2001 NH, 2007 Chevy Duramax
Also 11.3' Lance
Member Badger SKPs # 66377
Working as mobile employees for Camping World