Most motorhome passenger seats are recliners and might be more comfortable than the typical car seat although they won't have a footrest like a residential recliner. This should be easy to check out, especially if she has the opportunity to sit, or preferably ride, in one for a while. Lying in bed while on the road can be problematical, both with regard to safety and possibly a violation of state laws, which vary. In any case a motorhome makes it easy if you need to stop for a rest.
I don't know how riding in a motorhome would affect her anxiety. It does take some adjustment.
If the passenger seat isn't comfortable, and the MH doesn't have a suitable seat elsewhere, I think it would be possible to install a residential-style recliner, either in the passenger seat's place or in another location with a properly anchored seat belt.
You may find that a Class A will be more flexible with respect to the above than a Class C or B. Renting one for a day might help you focus on your needs.
You might want to find a MH owner in your area who would take you and your wife for a ride.
Whatever you decide, I suggest buying used so, in case it doesn't work out, you don't take a big depreciation hit like you would if you had to sell a new rig after a few months.
2002 Itasca Suncruiser 35U