wheel size-might be the dumb question of the year

Don't forget, from an engineering point of view the design has to call for managing unsprung weight of the tires and wheels. Larger tires and wheels on a light chassis ups the percentage of bouncing weight that chassis has to manage. It's way more than a cost issue.

Put your motorhome tires/wheels on you car and you can see what I mean. The tire bounce would take the car with it.

Usually the 22.5 wheels don't show up until the motorhomes cassis reach the 34-36 ft length, with a 18k+ chassis.