Here's my female/passenger advice for your wife.
Ride on the couch till you get used to the floating feeling you are experiencing when riding in the MH. Sitart sitting for short periods of time in the passenger seat. When you start to feel anxious or panicky go back to the couch. My hubby and I have been rving for almost 20 yrs now and sometimes I still sit on the couch when I get a nervous. :)
I'm driving our rig at least half of the time now and love it. Just give it time and you will become comfortable riding in the passenger seat.