Thank you guys.
Underneath the front part of the platform I can store 2 large boxes with the food, cooking gear, camp gear + a folding table (roll-a-table) and 2 chairs (roll-a-chair) + our sleeping bags + 2 wool blankets + 1 Goal0 extreme 350 battery + the 2 2 G water containers (Rotopax) you can see on the pictures.
Underneath the rear I can store 2 "trays" with the 2 burners stove and different flat items like camping pillows, shoes, more food etc. + a GSI micro table for the stove + 2 medium boxes with all the electronics, lighting gear, camera etc. + recovery gear + 2 propane canisters + electric cable + many little items I stuff in the empty spaces !
On the two sides of the front part, as you can see on the pictures, I put a small shovel and an empty MSR 10 l water container (not pictured) a small axe and a rubber mallet + 2 1 G water bottles.
So, on the platform, we leave our partially inflated sleeping pads (which are slightly bended with the seats in driving position) + some clothes (jackets, sweaters ...) + our luggages with our personnal stuff. When we stop for camping, we put our bags on the front seats, slide the seats in the forward position and we are ready to set up camp !
I have also made some kind of insulated shutters to put on the windows (inside) with suction cups. The idea is to insulate from light when sleeping but also from cold or heat.
We have used this configuration with below freezing temperatures: it's not really confortable but it's doable. The worst, as I said before, is when it's raining or snowing !