Hi Justin,

Few options.....

You could just get a very small electric fan heater.  Not much more power than the van heater but you could position it where you want.  This will be super cheap but will take alot from the battery and from what I've read on other forums you'ld need to be careful about how much else you had plugged in at the time to stop fuses blowing.  Running it off a second battery would be a good idea.    

There are gas heaters, convection or radiant that are cheaper than the propex one, say £100-£200.  The main problem is the exhaust gases are not sealed and vented out of the van so there is the real risk of carbon monoxide poisoning.  I have friends who do have this type of heater though.  I reckon it depends on how you will use it.  I'd never leave it on overnight but if installed and maintained properly and just used to heat the seating area during the day when you can keep an eye on it and make sure there is a window open for ventillation then it should be fine.

Lastly, a mate has actually fitted a wood burning stove in the back of a big old van they have.  Seriously!  I wouldn't recommend it but it is kind of funky......