Nice truck and cap. I'm on my second "pickup/cap" rig and have found this choice of truck/cap to be very practical, simple and versatile. Things I have learned:
For a metal cap insulation is very important - even a single layer of Refletex, sections of sleeping pads (the blue cheap ones from Walmart are great for this), carpeting or other similar padding/insulation will help with temperature management and sound deadening.
Add some powered ventilation, such as a roof vent or at least a simple powered fan.
You will need some LED lighting, and think about both internal lighting (general and reading) and some area exterior lighting - very useful at camp, especially at the rear doors.
It is very nice to have at least a minimal "table" set up near the entryway...a portable table or some type of door mounted drop down or slide out work surface that is lighted. Add a portable chair and your set.
I have opted to forgo built in cabinets/platforms as I use my truck for several roles...so I focus on versatility. I sleep on a Roll-a-Cot; this brand cot has no middle leg - so it goes up tight against side bed wall with no conflict with wheel hump, a Thermarest pad and a sleeping bag compete the bed setup. My clothes & gear go in plastic storage bins that are strapped down...dustproof, stackable (inside or outside), mostly weatherproof. I can go from an empty bed cargo mode to fully rigged and loaded for camping in about 20 minutes.
Good luck with your project!