Lots of potential issues here,
83yr old mother, even if spry makes me wonder, DW's mom wanted us to buy another C because she thought she could lay down and sleep all day while we drove. Trying to lay on a bed in a moving vehicle is not easy. Climbing in and out of the rig also will become a strain. Also everything else takes longer, and with 4 adults and one very small bathroom thats going to get tough too.
You can be a close knit family when you start, but its going to be a major strain trying to make this work.