My family rode in our truck camper any time we went anywhere, probably 30,000 miles. I could not keep them in the cab. I had a boot and kept the passage open most of the time and unlocked all of the time so communication was not a big issue. The camper was much more comfortable than the back seat of my supercab p/u and it had the TV, fridge, toilet and beds. I would do it again in a heartbeat. I have recently upgraded to a class A and really don't believe that it is much safer but seems to be much more socially acceptable.
The only "injury" case I have heard of was about 20 years ago in Birmingham, AL. A man was sleeping nude in the back while his wife drove. He was awakened by the vehicle stopping and for some reason peaked out the back to see where they were. At that moment the light turned green, his wife stomped the gas and he tumbled out into traffic.
The only problem I had was I stopped for gas late one night and I went inside the station to use their restroom to avoid disturbing the sleeping kids. Unknown to me, one of the kids went in at the same time. I came back out, saw that all was quiet, fired up and hit the road. About 5 minutes later I got a call from the gas station wanting me to come back and get my kid. Thank goodness for cell phones.