I have a 2007 River Canyon that I purchased a few months ago with the same issue. I was able to remove some with a soft brush and a liquid dish soap/water mix. I am in the same boat as you though. I could not get all the streaks removed, especially, from the side edges of the window and water heater/refrigerator vents. I hope we can get some ideas. I was thinking maybe an automative fiberglass cleaner/polish.
Dennis & Estie
2007 River Canyon 31RLSS Travel Supreme