Hello Remco,
Good looking Idea!
Be careful you put not too much load on your rear axle! very Important!
Also the rear spare tyre, have a look how much distance it is from the centre of the rear axle, and weigh it, than you can calculate the dynamic weight. then you might decide to place it at another place.
do you put the new camper over the existing sheet metal in the back, or do you cut of the existing sheet metal and replace by the new camper?
be sure that you do NOT pass over the GVW with your camper and personal stuff.
you really need a bit of reserve.
I went into Libia with a '77 Dodge W-200 and I took 1600Kg of camp stuff and spare parts, after 5 weeks and roughly 15 000Km further all the sheet metal from the bed was gone, only held together by these heavy duty rachet straps.
That's how I know.
good luck!