Jason,
We bought our first rig when our first child was new born and kept that rig for 24 years. Critical for having a good time was to have the motor home with us all the time. Leaving "The House" back at the campsite does no good with the needs and comforts with young children.
My point here is, get a rig as short as possible that meets your needs with your child, and assuming one day you may have another child or two. I wouldn't consider anything so comfortable & long that you can't get around adequately in national parks. I'd sacrifice some interior comfort for improved mobility.
This coming from a fellow who travelled to national parks often, with wife, two kids, and an occasional guest, in
THIS THING for 24 years.
Back at the time, seat belts for motor homes were not yet required. I shamefully confess that as the kids got too old for a car seat, everyone in back just sat on the bed. It was the mentality back then. I would never think of doing that today.
For a child seat today, I would add a "D" ring bolted through the floor to anchor a child seat better into any of the RV seats in back. As with all new cars today, the seats belts are not enough. New cars have that extra critical anchoring point.
Actually the safest place to mount a child seat in a class B+ or C is the front passenger seat. Let Mommy or Daddy sit elsewhere. We did that for a while in our old rig. We tried numerous configurations then, even anchoring the child seat into the floor between bench seats, just behind the cab. But when the second kid came along, our options became very limited with the oldest child already 6 years old. Still we could have done better.