I'm very late to the party.Sorry!
With respect to the overhang,I suspect,that if you cantilever out beyond the windshield header,that the underside of the overhang will begin to act like an inverted splitter,directing some air under and to the sides rather than up the sloping nose of the camper.
Some of the centerline flow will displace far in advance of the shell/cab roof intersection and just go on up over the top.Streamlines below there will be directed sideways around the A-pillars and up into the void between roof and shell.
The hard edges as many members have mentioned,are sure to produce immediate separation.Hucho will have two formulas you could look at,one based on a single dimension (say width),the other,on the squre-root of the frontal area of the shell.
These would offer 'minimums' for attached flow at zero-yaw.You want as much radius as you can stand to build.In crosswinds,this is where you'll save big time.I realize it cuts into interior space so you're the boss of that.
If you do decide to push the camper out past the windshield,I would recommend that you 'soften' all the edges around the 'splitter' area.There's gonna be some wicked transverse flow,a lot of shear,a mixture of velocities attempting to marry one another,and it's a breeding ground for vorticity which may cause so much wind noise,the weather stripping and seals around windows will be overwhelmed,making a loud stressful driving environment.
If you absolutely must use the hard edges you might get some relief with VGs,used just to break macro eddies into micro eddies,altering the frequency to something you could live with inside the cab.
I used to drive a 35-ft moving van with cantilever box.You could clearly hear the sheet metal tin-canning as the turbulence was beating the s--- out of it above my head.Very stressful.
