If you look at the Winnebago product marketing roof/sidwall construction example that Winnie has placed at many dealerships, it is clear that any water that escapes the supposed-to-be sealed edge of the turned-down roof will run along the aluminum extrusion that is the longitudinal part of the roof-wall joint. It cannot run *directly* down into the wall.
This is not to say that the water will not find its way into an undesireable place - it probably will somewhere. It could be at one end or another of the roof/wall joint.
2005 Horizon 40AD, 2005 Jeep Liberty CRD