The space in the roof may be needed to store the bed cushions when they are not in use.
I use the boating word 'galley' for the kitchen. In this version of the design the top of the galley sticks up 8"/20cm above the surface of the bed, so it must be removed at night. So this 8"/20cm makes a box (see it opposite the sitting person's knees in the drawing further up this page) and it is removeable. This has the extra benefit that you can take it out of the camper if you want to use the camper to transport big things.
Two people can sit in 49"/125cm, but it certainly isn't spacious. Go to McDonalds and measure the width of one of their four-seat tables and it won't be much more than this. The camper can be made a little bit longer, but it will always be a tight fit. Get used to it - it's a Mini!
The back will consist of the big hatch that opens upwards and then under this is a small door, hinged on one side, to make it easy to walk/climb in - it's drawn below. I don't think the pick-up tailgate can be left on - it won't be able to be closed (the camper is in the way) and it will take away half the standing area in the tent (it is shown dotted below).
There is not much point building the camper with double-skin insulated walls as you will be sleeping with your feet in the 'tent' - this will not be a winter camper unless you are tough. So I would use a single layer of thin plywood, either 1/4"/6mm or even 3/16"/4mm, with a framework of 1"x2"/20x44mm wood stiffening. You can either use exterior ply (contreplaqué extérieur en francais) if you want to paint it or marine ply (contreplaqué marine) if you want to varnish it so that the wood grain is visible. MDF is out - it's heavy, weak and falls apart if it gets wet.
There are several extra details I need to look at in this design - I am not happy at the moment that the rear end of the camper can be made strong enough. But the first step is to get some accurate measurements of your actual pick-up and its wood lining - I will send you this in an e-mail.
Andrew