Greetings & Welcome!
I’m a cold weather camper, and it’s currently -15°f where I am. Aside from the possible icey/snowy road conditions, you need to figure out your plans and whether you want to heat the van while parked. Running your engine while parked is a poor idea.
Any time it’s below freezing I heat my van with a kerosene heater. Some people use propane, but it’s just to dangerous in my book, plus it can be finicky in cold weather. I also have 12v heated blankets and seat cushions. Between them, I can stay comfortable regardless of the weather.
Even with all these things it’s important to have clothing and bedding to keep you comfortable without any of them. That includes head wear, footwear, and gloves. Have plans, backup plans, more backup plans, and an emergency plan. I have multiple heaters/cookers, and months worth of kerosene. I could get snowed in until spring and still live comfortably and eat well.
West of the Cascades stays warmer, and doesn’t get too much snow. The high desert can get pretty cold and windy. The mountains can get snowy, and most lesser used roads are likely to be unplowed. Most of the parks and campgrounds will be closed. Many of the roadside amenities such as rest areas, gas stations, restaurants, and even grocery stores might be closed for the season. Stock up well while in towns.
"Screw the insulation, give me HEAT!" ~ Hobo Henry