Greetings!
I’ve done winters down to -60°f, and I can tell you that you want a window van, on a sunny day those windows can provide 40-50 degree rise in inside temps. Insulation really doesn’t help much because you need constant ventilation, and it prevents that free solar heating. What you want is a big heater, at least 20k BTU’s. Between my heater & stoves I have about 60k BTU’s which is really nice to warm the place up fast. My uninsulated window van takes very little heat to maintain the temperature once it’s comfortable, but raising the temperature can take a very long time if you don’t have enough heat.
When I had a heavily insulated cargo van, it cost me many times more to heat than a no added insulation window van, and the cargo van was always cold and damp no matter what I did. Windows, ventilation instead of insulation, and more heat is what solved the problem. I switched from propane to kerosene for both heating & cooking. Cheaper & better. The kerosene heat also requires no power. I put a heat powered fan on top of it to circulate the heat, and no house power is used at all.
Unlike the propane, I am able to safely run the kerosene heater all night, of course you should still have a carbon monoxide detector just be safe. I also have 12v heating/cooling blankets & seat cushions from a truck stop. Combined with my heated/cooled rechargeable coveralls, and self heated boots & gloves, I’m ready for anything.
Cargo vans make horrible campers, especially in extreme weather conditions. Opening windows add so many advantages, a $20 fan can provide better ventilation than 2x $250+ roof vents, and you don’t need to add holes in your roof. Free heat in the winter and free cooling in the summer, neither of which requires power. A view, and much lower insurance costs don’t hurt either, not to mention the higher safety standards for passenger vehicles. They’re also much cheaper to convert, just remove the rear seats and move your new furniture in because the floor, walls, & ceiling are already finished.
I still think factory RV’s and camper vans are a better choice for most people, especially newcomers, and older ones can be found pretty cheap. Only many years of actual experience will give you the knowledge to build your own. The learning curve is much easier if you start with a factory camper. Most newbies go through many builds and tons of money before they’re actually happy, because they didn’t start with a factory camper.
Cheers!