In the old days of chrome bumpers, we used to remove rust from bumpers using Coca-Cola. It's the acidity in the coke that does it. I guess vinegar would probably work as well, for rust that is.
BTW, looking at the picture of your roof it looks like there are water rings or high water marks. I would check if the roof is concave and if it is you may have to add something to the inside to restore its bow so that water will drain off.
I also notice the luggage rack. Maybe the PO carried something heavy up there or at least heavy enough to cause it to sag.
Or maybe a water leak around the vent caused rot in the plywood under the skin. I would think that that should be repaired before taking it back out in the weather.
There is a discussion somewhere on this website about that problem.
A charter member of the Buffalo Plaid Brigade!
Whether you think you can or think you can't, you're right.