Hi!
Not sure about a 12V one, can't imagine much umph with that, and if they are a high wattage, you'll need to upgrade the cabling to the fag lighter. Only 500W is 40+ amps. And it'll flatten the battery by the time the van has heated up no doubt
http://www.ohmslawcalculator.com/ohms_law_calculator.php" onclick="window.open(this.href);return false;
Would you be using the heating on a hook up on site? If so, would this do:
http://www.ebay.co.uk/itm/Plinth-Heater-Built-in-or-Built-Under-2-Kilowatt-Electric-Plinth-Heater-/151170306722?pt=UK_Home_Garden_Hearing_Cooling_Air&hash=item233273d2a2#ht_2635wt_1141" onclick="window.open(this.href);return false;
13A plug, 2kw's is 9A @ 220V, so fine for a lower current on site hook up, i think they tend not to be 13A, so i have heard. I am off grid with solar, so never used on site
A propex is great, i have one, highly recommended, but very pricy compaired to the above. Depends on how you are likely to use the van
Hope this helps
Cheers
Alistair