Now I could start on the inside.
First step was to thouroughly clean the floor, and cover all exposed metal with paint (Hammerite) to prevent rust.
Once that has been done, I installed some pine lumber and glued it with Polymer adhesive (Den Braven HighTack). Since I needed to have as much as possible height available, I tried to limit the floor height to approx 2cm (0.8″). I didn’t go any lower since I still wanted to have at least some level of insulation of the floor.
Also, this allowed me to run some pipes for cables from one to the other side of the van, including 2 wires from the starter battery to the leisure battery for charging.
On the entrance (next to the sliding door) I added some more lumber to better distribute the load on the floor.
I also cut the nessecary holes in the van for the gas vent and the Diesel heater (I’m going to place it in the back of the van).
Once the wood was in place, I installed all piping (and included the wires, since it was likely to be a pita to insert the cables afterwards). Used a bunch of Polymer adhesive to secure all piping.
Finally, I covered all spaces with 2cm thick PIR isolation boards. This is pretty rigid stuff so it will give the floor some more support. I screwed the floor (9mm plywood) into place and my floor was finished.
I did go for PIR, since it has better insulation value (Rd 0.9) and is more rigid, thus less supporting structure needed compared to eg Armaflex or Xtrem (Rd of approx 0.6 for 20mm thickness)