Greetings & Welcome!
Insulation is way over hyped and over rated when it comes to campers. Only good heating and cooling choices will keep you comfortable, with or without insulation. I’ve got an uninsulated high top window van, and frequently spend winters where it’s cold (-57°f this last winter), and summers where it’s hot and sometimes humid too. ONLY good heating and cooling options will keep you comfortable, insulation alone will not.
Windows offer a view, great ventilation, and some free heating and cooling as well. Much better than roof vents that inevitably leak, and a ~$20 fan will do you much better than two $200+ roof vents.
Access between the cab & cabin is also a big deal, and most freezer trucks don’t have this. For both safety and convenience, you don’t want to be forced to exit the vehicle before you can drive off.
After years of experience, I recommend a ready to roll RV, or a passenger van that you can simply remove the seats and move your new furniture in. No building of floors, walls, or ceilings, or the unnecessary time and expense.
"Be the reason someone smiles today!" ~ Van_Dweller