I'd bet that manufacturers select the chassis primarily based on its Gross Vehicle Weight Rating so that it can just provide adequate cargo carrying capacity. And also that they select the wheelbase, whether available from stock or having it stretched, to a length that will more-fully utilize axle capacities. Much beyond that I doubt American motorhome manufacturers know much about incorporating handling in their design.
Jamie, to your point, I would also expect all Axis/Vegas to handle similarly because their sizes are very similar, although there are likely some minor differences due to the way mass is distributed within the motorhome. Based on floorplans, some models appear to have greater polar moment of inertia than others. That's not a deal breaker but may influence handling enough to notice between models.
The bigger difference in my opinion is when motorhomes of similar lengths are built on vastly different wheelbases. They can't possibly handle the same.
Height is also a factor affected by the motorhome designer that is beyond the chassis' control. I can't think of a single case where added motorhome height is good for handling. The higher center of gravity, higher center of pressure in crosswinds, etc. are all negatives.
In summary, I think motorhomes are designed more like if done by architects than engineers (not that that is all bad
). I believe design of the living space takes priority over how the motorhome drives under less-than-ideal conditions.
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