Larry N. said:
It's not nonsense. Most of us who've had such coaches have actually experienced it, many times. If there were no resistance from the two axles, then your thoughts would be right, but you need to take that resistance into consideration. The axles do mimimize that effect for where they are located, but they are not evenly distributed along the length of the coach, thus the rear has less sideways resistance than the areas that have the axles to support them. In many ways, though, it's worse when a truck goes by, since the sidewash off that truck isn't hitting you evenly, rather it starts at the front, where the front axle minimizes the effect of the push, proceeds to the middle where the rear axle does the same, then finishes at the rear where there is nothing to stop that effect.
But let's use the exaggeration effect (often a great tool to help in understanding a principle), and change the configuration of the coach (assuming that there is still a reasonable weight balance) such that you have 6 feet in front of the rear axle and you have a 27 foot rear overhang. Does that help you see that a wind will push the rear around?