I was considering a toy hauler model, with a fold down rear ramp. One model by Heartland has a small bathtub and toilet in the back area, as well as the normal larger bath that would be up a few steps in the front area.
I don't know how much I would modify a fifth wheel to accommodate the wheelchair. I have seen some lifts that are mounted just inside the door, and the wheelchair lifts up with a sort of screw jack, up to the floor height, then is swung into the RV. So just about any door can fit the wheelchair inside.
Good luck on your RV selection!
Fred.