Stubborn streaks on motorhome.

I bought a second hand fiat ducato swift sundance 2006 motorhome last october.  I have washed it today for the second time and just cannot get rid of streaks.  I used Autoglym caravan and motorhome cleaner and it just would not shift dirty marks.  I have used this in the passed when I had a caravan and it worked wonders.  I wondering if somekind of wax or polish has been used in the passed and this has effected the Autoglym.  Any ideas how to get rid?  Very frustrating!!