The 21QB is on the 158" wheelbase E-350 which is a stock Ford configuration. It comes with the 5.4L V-8 standard. The 6.8L 20v V-10 is optional from Ford. The 22QB appears to be a non standard (coach builder modified chassis) at 168". My guess is that Coachmen is using the optional V10 on the longer chassis due to increased GVW. In other words, what Grumpy said.
The recipe for perpetual ignorance is:
Be satisfied with your opinions
and content with your knowledge.