Need More Heat!

I use a few 2000ish E250 vans for transporting handicapped people. It seems to never be warm enough. I was wondering if anyone has tried to put some sort of electric heater in these big vans? E250 van is so hard to keep warm just simply due to the cabin size. Obviously would need something that was for a 12 volt system. If anyone has acccomplised this or knows where to purchase this type of equipment let me know. I have seen the cab heaters that can be mounted and run off coolant lines but again those only heat as fast a the vehicle does, but dont seem to keep up and when the doors are opened and people are getting in and out it seems to take an hour to restore heat back in the vehicle.

Generally a van alternator cannot put out enough electrical power to do some serious heating.

I would suggest, if the vans are running for long periods of time, a hot-coolant heater is still the way to go, just get a large capacity unit:
http://www.accclimatecontrol.com/heaters/20059A000A.htm

If trips are short, perhaps a propane heater may be usable,
Try this site to see what could be used.
http://www.camping-heaters.com/propaneforcedairheaters.html
Any propane heater would have to be exhausted to the outside, though.