Parking lot with a hook-up

[Ken-55]

Here's one that I'd recommend:

Newegg.com - DeLonghi EW7707CM ComforTemp Electric Oil-Filled Radiator

Regularly $70, on sale for $60, using coupon code, EMCYTZT976, on check out drops it to $48 with FREE UPS 3-day shipping AND there's a $10 mail-in rebate makes your final cost $38.

It has three heat settings which draw 700 watts, 800 watts or 1500 watts and is thermostatically controlled with an "anti-freeze" setting that only kicks in to keep the area above 42 degrees to conserve energy.

I have nothing whatsoever with the company making or selling this, I actually have a different brand, similar style heater, I just think that this would solve the OP's issue.

[WA4SLC]

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