Which RV have the best aerodynamics? A, B or C?

They're all so terrible aerodynamically that it just doesn't matter which one you choose.
Get what you like.

Hi,

Class C is the worst from an aerodynamic point of view.

How deep are your pockets?

If trailer companies wanted to make a more aerodynamic shape they would be better to round the back of the trailer. Most people don't understand that putting the air back together is more important than how you rip it apart.

This makes almost zero difference in MPG, as your tow vehicle "hides" the bottom part. Unless this is a theory exercise, this would be the absolute last thing to consider about rig choices.

Weight and if it can be towed safely by your tow vehicle first, floorplan, comfort, and ease of use/living is second, fit/finish is third, color and trim, after that.

It's more important which one has the layout you want for your RVing lifestyle UNLESS you are on the highway more than in a site.

Negligible differences....