Just wanted to provide an update...
I called the tire place that installed the new Yokohamas (Tire Man
in Thousand oaks, CA), and Jared was nice enough to meet me at my RV, rather than asking me to get it out of storage and bring it to him.
He showed me that the tires were indeed mounted properly on the wheel, and that the slightly different curvature of the sidewalls exposed a little more of the wheel rim than the Goodyears did.
The reason that the gap appeared uneven is that the wheel cover (or "wheel simulator") was not exactly centered on the wheel. The holes in the wheel cover (for the lug nut posts) are larger than the diameter of the lug nut posts, so there is some slop there.
He said the nuts that hold the wheel cover need to be loosened a little bit, so that the wheel cover can be shifted 1/4". He offered to do it the next time it's convenient for me to stop by.