I've always worried that airline passengers will look down on my fiver roof and think I'm really trailer trash. There's nothing worse than brown stains running away fom the A/C off the radius of the roof onto the aluminum extrusion.
If Dawn won't take it off, ProtectAll or Camco Rubber Roof cleaner usually does. I've used both on the A/C stain with a bristle brush like you would use to clean your tires or floor mats. On any of my 3 Montanas, I've not had any rubber roof failure due to a bit of scrubbing. Aerospace 303 after a good scrubbing helps lessen dirt and stain buildup for the next go round.