I've seen the bumper air tanks on trucks, (friend has one), they are very cool idea, work amazingly well. Never seen one on a van though, also I don't think you'll get much air in nurf bars though.
I've read very good things about the Puma 12v air compressor, comes with tank and everything you need for a very good price, then you just separate compressor from tank for install.
http://forum.ih8mud.com/camping-outdoor ... essor.html
I did read some comparison tests somewhere, and it was better (fill quicker / better recovery time) than the Viaair and Extreme air, but I have no personal experience with them.