BS.
There are only two ways of doing this.
Method one. Webasto heater that uses diesel
Method two. Gas heater that use propane.
Method one costs £2000 and £10 a day to run. Method two costs £300 and £1- £2 a day to run.
Method two is very simple, but because of safety issues i don't believe it's legal to give advice. Nathaneal has a very informative post on here about it, and of course his Ebook that's free to guild members.