I think it is a optical illusion. One gets the same feeling in a class 8 cab over truck, and even in a class 8 conventional truck. Even the driver can get that feeling. One either learns how to deal with it or gets use to it. At least my dw says she has gotten use to it. Tell your wife to hang in there. She is sitting in a spot where there is no reference such as a mirror to view down the side of the coach like you have. She feels like the passenger side wheels are driving down the shoulder and she is sitting on top of one of them. You could mark the windshield with a piece of black tape to make it easier for her to reference where the actual edge of the road is. Or set up a spot mirror for her to view down the side of the coach. Some drivers have been known to do that on the drivers side.
I driven tractor trailer most of my adult life. My first class C motorhome was a Toyota Sunrader. I remember the first time I drove it and looked in the mirror. That fiberglass box behind the cab looked absolutely huge compared to the little Toyota cab I was in. Gave me a most uncomfortable feeling. And I have driven two trailer "b trains" grossing over 130,000 lbs.
Looked like this, but add another axle under the front trailer. And the dolly pulling the rear trailer is attached to lead trailer with a pintle hook, so you end up with 3 pivot points on that rig.