It sounds like chalking to me, totally normal. I use a non foaming detergent on mine, like TSP or Spic N Span. Check your owners manual for recommendations on roof treatments. Mine specifically warns against using them. Start washing on the roof and rinse well. Washing from the bottom up on the side walls makes sense, but if you wash the roof last you will have to rewash the sides because of all the chalk from the roof. Washing it twice a year works well for me. It keeps the black streaks to a minimum. Keeping the side walls waxed makes for easy removal of the black streaks if you wash the side walls on a regular basis. You may also see some little black dots on the roof after washing. Also totally normal.
