Lots of people who have never though of or done this conversion seem to have an opinion for you, the consensus of the great-they is that you are wrong! You are wrong to consider spending time in your camper, rain or not. You are wrong to make it more livable. You are wrong in all of your thinking.
Haha, Im just having fun. Don't take this seriously.
That conversion sounds like a great idea - a place for the kids to be able to sit at a table and be inside without crowding everyone around one table, and for the kids to have their own area also. Maybe they will need to sit on the floor to have headroom. If you had 4 bed cushions, like a dinette, instead of one big mattress, then they cold stack 2 high side by side and make a cushion table in the center.