Not trying to be pest here Malcolm, That if you were to drive threw an older, neighborhood housing development. That you would notice that there is no roof overhang from the side walls. That in later years the overhang was developed, to keep the melting water from backing up from the lower roofs ice jams at the roofs lower edges. In the winter from sudden weather warm ups, and flowing past/under the shingles and into the walls. Instead, with the roof overhang, when this occurs as it still does, the water gets past the shingles/plywood underlayment, but drips down & out from the bottom of the overhang, The rafters are notched to sit low at the tops, instead of sitting on top of the upper plates, this notch is called a "Birdmouth", in the trade. Thus allowing in the roofs slope, the overhangs lower horizontal bottom half tthat comes back into a/the finish wall at a lower point. Thus any water at this point meats a vertical finish wall to drain down. And on a house, any water problems with guttering/fascia board. away from the walls the depth of the overhang. I had a roofer foreman tell me once, that you can control water by the flow of the water, far, far better that to try and stop it.
Thus when I remember the older truck campers over hang, the two came together. Now whether this would apply to your class c traveling 60-70 with a head wind..........????? Your choice of course !! In your spare time, (pun intended) you might sketch out several potabilities. Key word here "POT", Only trying to pass on possibilities.
John H