Greetings & Welcome!
Building your own camper van is usually bad advice, and will end up costing you much more than purchasing a used factory camper, and not be nearly as usable or comfortable either. Generally you will lose the cost of any additions, and in this case you might destroy the value of the van as well.
Camper vans REQUIRE ample ventilation, which will destroy the usefulness of your refrigerated van for anybody else. Windows also add greatly to the camping experience. Bad weather and other things happen, where we want to camp out inside our vans for multiple days without leaving. Would you be happy in a windowless box for multiple days? Most people wouldn’t. Adding windows could get terribly expensive, and defeat the whole purpose of buying a refrigerated van.
In my experience, only good heating and cooling methods will actually keep you comfortable, and this is true with or without any insulation, and I am often an extreme weather camper. I’m currently south of Miami, and without my swamp cooler it would be really miserable here, and no amount of insulation would change that. The same was true last winter in Minneapolis and Duluth MN; good dry heat kept me comfortable, not insulation, and it got down to -60°f.
Cargo vans are also not stealthy, they have that “Creepy Cargo Van” stigma attached to them. Stealth isn’t about hiding, it’s about appearing to be harmless and being a good neighbor. With a cargo van I was judged by my van, with everything else I have been judged as a person.
Cheers!