What is the minimum clearance I need between the truck cab and camper overhang?

I’d say not may have the “exact answer”. 
1.5” seems a bit light, but may work. Probably fine under normal conditions but hitting a big dip or frost heave at highway speeds might test that theory. I’d shoot for a bit more. Nothing a sheet of plywood or 1” foam under the bed mat won’t cure. 

congrats on the new rig!