I have driven vans that squeak like a load of mice.
To prevent the plywood and slats from squeaking I used bonding cement to attach the slats to the valleys that brought them up to level with the ridges. I also used some insulation between the slats and cabinet grade 3/8" plywood. The van existing tie downs were used to secure the plywood to the floor.
I really didn't know how important it was to have good insulation in a camper van. Just for the record it's very important!
Prior to insulating, the van was like an oven because of the black metal and radiant heat. After installing insulation it was amazing how much cooler it was during the build. Do not skip this step. I've see many ways of insulating vans on Pinterest and YouTube. I choose to use closed cell 1" pink foam insulation. It comes in 4x8 panels and is very easy to cut to shape and the cut edges do not shed loose pieces of insulation. I was able to keep the panels in place with flat brackets screwed into the van ribs and overlapping the foam.
Ok, I overdid it a bit on the electrical system. I looked at many fuse/breaker panels for RV use and they all look like cheap half effort.
I have 12 and 120 volt power needs. A "Blue Sea 8084" panel allowed me to include both in one panel with top of the line circuit breakers. The 12 volt is 10 amp and the 120 volt is a 30 amp service. I set the whole system up in my work shop and ran power to and from the panel. You can see three light bulbs on top of the power cabinet. One on the right is 120 volt 200 watts, the center bulb is 120 volt 60 watts and the left bulb is 12 volt 10 watts. I ran this for a week in the workshop to confirm the connections and the charge-discharge cycle work like I had designed.
It all came together including the granite cabinet tops.
I found a remnant that is 1/2" thick. During the entire build I was concerned about the weight I was adding to the van. I also wanted to keep the weight low so it would not be top heavy. If I were an engineer, I would have known all this before I started.
I had a local RV sales/repair company cut a hole and drop the AC in place.
I had already built the frame and wired the electrical. All they had to do was hoop it up. It is a Dometic Penguin low profile unit. I was told by the Florida Dometic rep that I wouldn't need any additional sealer or gasket on the top of the van. I was very leery if this would work. The top is corrugated metal and is anything but level. It did last for about a year. After that no amount of tightening would prevent it from leaking. I had to remove the AC unit and level the gaps out with butyl tape. All is well now.