why so few 7'6" (or less) coaches come on chevy & ford cabs

I've seen some quite puny OCCC ratings on Sprinter chassis based "C"s when at the last RV show a few months ago. Sprinters are nice rigs. Especially when it comes to the cab doghouse department.

Phoenix Cruiser makes some decent rigs on the E-450 chassis, and they are marketedly slimmer than other makes. If I had the cash for a custom "C", I'd definitely look at what they offer. With a slide-out or two, the 6-12 inches lost in width wouldn't be noticed.