So what are your plans for heating a cooling? Or are you just doing it for noise?
The thing I figured out after a while, is that unless you plan on installing some sort of heater, and active cooling, then there is little reason to insulate other than for noise. If parked in the sun, it will get hot fast, and stay that way. I had just a thermostat controlled vent fan, and I would regularly cook during the summer. Same problem in the winter.
Unfortunately, the diesel heaters you can install that are vented properly are bucks, but worthwhile if you're really going to make it into a camper. For cooling, there are basically no options unless you're going to plug in somewhere, or run a generator. Even with a pile of deep cycles all charged up, you probably couldn't run AC long enough to get you through a day.
Might want to look into some of the RV based gens that can mount under the back of the van. Probably the best way to go, even if the initial price tag is rough.