I'm sure its a mix of all three: hopefully your #3 doesn't contribute much. There shouldn't be much variance from unit to unit especially with the chassis. The house portion is largely hand built and thus would have the most changes.
I'm guessing that the manufacturers (Thor, Winnebago, etc.) select a chassis based on a weight range and vehicle size. The drivability or not simply comes along with the chassis--its not like they design or account for how it handles.
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