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

I know what you mean.

When following behind a tall Sprinter motor home, it looks like a gust of wind will lay the rig over on it's side. The pairs of rear tires are placed much closer to the center than any other DRW in the industry. It actually looks quite strange to me. Imagine taking off the outer two rear tires, and placing them on the inside of the inner two tires. It's not so, but it looks nearly that extreme.