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

Theoretically C would have the best aerodynamics, but the truth is you'll need a lab and the difference will only exist on paper. The most aerodynamic shape would closely resemble a double edged needle. You're gently separating the air over a longer length and gently allowing that air to return to its original state. This allows for a less turbulent airflow which means less drag. Trailers are no where near this shape. A conventional, v-nose, or rounded front cap is still a very large object being pulled through the air. Front caps are only a small part of the equation. The flat rear of the trailer has just as much impact as the front cap. The flat surface and sharp edges causes an instant void which air rushes in to fill. That rush of air creates drag. Airstreams does a better job since their caps are more rounded, but they're not the end all be all of efficiency. There's also 3 other areas on trailers that have impact that people forget about: the roof, undercarriage, and protrusions. Until we see some radical changes in the aerodynamic engineering of these trailers, they're all essentially bricks being pulled behind vehicles. It would be best to focus on floorplans and find what makes you happy.

Rockhillmanor mentioned air deflectors, but they aren't really that effective in the real world. The deflector has to be placed fairly close to the trailer and angled properly or the airflow will dip between gap the tow vehicle and trailer making it ineffective. In most cases it's not possible to get close enough due to the tongue length. If you were able to get the deflector dialed in perfectly, it would still only addresses one direction of airflow to the trailer. Air still enters the trailer gap from the sides and under the tow vehicle.