I would say that just a drop cord through a window with the heater that Ken-55 suggested or something like that would work. You did not say if your van is a cargo van or passenger van. If it is a cargo van, you could get a roll of space-blanket insulation from a hardware store and insulate your van also to keep the heat in and make it more comfortable. You can power you laptop with the same drop cord and a multi outlet attachment if you keep the heater on a lower setting.
Good luck. I hope it works for you.
Bill & Camillla
Thor Four Winds 31L Class C and Jeep Liberty