I had a 1982 Dodge camper van (called Señor Majestico) and spent a lot of time in there a few summers ago. We were even skiing the whistler glacier for a few days, so I feel like I have a little bit to say aside from the Will's wisdom.
The van is going to get dirty very quickly. Keep it clean. It's probably a good idea to have a broom and dustbin system, or a dust buster (if your inverter can support it).
Ski socks and baselayer stink. Bad. It's going to be hard to dry them out as it will get very, very damp in the van if you're in there during the winter. If it gets too bad, it might be a good idea to put them in an airtight container until you can launder and dry them.
Does your van have an inverter? If not, get one installed or at the very least have one you can plug in. You're definitely going to want power.
You can eat microwavable burritos cold.