OK, so to establish some basics:
It needs to fit one person comfortably for 2 to 3 months and two persons comfortably for shorter periods (assuming you'll be visiting during those months and you'll be taking shorter trips together at other times).
You don't want/need a truck or SUV.
The property has a house on it and someone living there full time to watch and care for the RV.
Some additional questions:
Black and grey tanks - is there a sewer clean out or similar on the property to make dumping the tanks easy? Or will she only be using the RV to sleep and will be using the house facilities for everything else?
If she'll be using the RV facilities, then who will be dumping the tanks when needed? You'll need to factor that into your decision. Showering will fill the tanks sooner than just using the toilet - using the house shower will lessen the frequency of dumping the tanks. If there is no way to dump on the property, then someone will have to be able to take the RV to a dump station when needed - will your wife and/or daughter be able to do that if you're back in Florida when the tanks fill up?
How much of her time will be spent IN the RV versus in the house with your daughter? Would her disability affect her ability to do things like convert a dinette or couch into a bed, use a small RV shower, etc.?
A true class B could be a nice choice. Your daughter could "exercise" it on a regular basis in the off months. But it may be too small for your wife to be comfortable if she'll be spending a lot of time in it during her visits.
A class A would probably provide the most usable space for her. But if it has to be driven to a dump station at regular intervals, your wife and/or daughter may not be comfortable driving it.
A class C (or B+) could be a good compromise. If your wife/daughter is comfortable driving a B, then the C won't be much different. A 24-28ft rig could provide a walk around bed and still be short enough to feel comfortable driving it around if needed. It would provide for a good amount of indoor storage so things could have "their place" and still be easily accessible.