My insulation thinking has been mainly for noise. If I really need the cooling, I'm thinking go with an top RV A/C unit, or a fan. Of course, with an AC unit, I may need to pay attention to highway overpasses. Needing an AC for summer heat could be a problem, but then that's a $40-70 hotel room with shower problem.
I have no delusions about batteries and AC units. I have found, that with the pink styrofoam, it would stay cool inside. Of course, that's not opening it up during the heat. Using the van as a Pit Shelter or open tent would change that completely.
I've seen the prices on those ESPAR diesel heaters, and they're evil high. I don't really plan on really cold camping, and if I'm cool, I do have a 12v blanket or a good sleeping bag.