Hard water stains

Yep, as Bill said, condensate is not hard water. You need to figure out what smutz the water is picking up from the a/c and roof as that is what you're seeing when the water dries. Sometimes it's just the wax reacting to water staining, which is fixed through the use of a cleaner wax.

If it's condensation from your A/C, that would not be hard water. Condensing water is about as soft as it gets as it's nothing other than what's in the air.
The water then drips and runs across the roof picking up junk along the way. The rubber roof's are most notable for causing the white streaks on the sides of RV's. Once you get the sides clean, keeping the roof cleaned and sealed is a better long term solution.

Don't know if it will work but how about Mr. Clean magic eraser?
Clean out the gutters and raise the front/back slightly higher to allow the water to run off.
For the Refer drain can you put a spacer between the hose and wall so the water doesn't run down the wall.
P.S. we don't seem to have this refer problem.

JoeH wrote:
clay bar will do it

Yep!

clay bar will do it

Muriatic acid followed by washing with soap and water.
Wax to reduce future staining.

Try a little vinegar on a rag.

We've had this and used various things, not found anything real good yet. I don't know if you would be able to use CLR or some other mineral deposit cleaning agent on the paint or not - maybe ask a body shop?