Yep.. I spent one NORCAL winter in a van with zero insulation, not even wall paneling. Just two throw rugs on the bare metal floor with an Ikea brand futon mattress ontop. It was utterly miserable. I woke up just abut every morning damp from condensation, mold was a constant problem, and I had a small room heater I could use when I had access to shore power, but whenever I used it, it just made the condensation problems worse.
All I had in that van was: My clothes, a room heater I barely ever got to use, a 0-degree sleeping bag, a battery box with inverter for my coffee machine, an MSR camping cooking pot, a stainless steel coffee mug and a gasoline fueled 1 burner cooking stove. That was it... Not even a port-a-potty..... The conversion van windows leaked like crazy, and what broke the camels back was waking up one night during a rain storm choking on water I inhaled in my sleep from a SOAKED pillow from that leaky window. Then discovering that mold had started to grow ON MY DOG's HEAD.... That was it.. I couldn't take it anymore.
The next morning, I bought a used class-b RV. Best cash I've ever spent.. I'm in the lap of luxury now compared to that winter.
But, I learned ALOT from that experience. I know whats possible, and I can do it all again if I had too with one important thing... A dehumidifier.... If I had one during that winter... I'd probably still be living in that van..