I see you have not had a reply and I can't help although some new tourers have a water or antifreeze central heating system.
You are probably posting in the right place but may have more luck in
http://www.ukcampsite.co.uk/chatter/display_forum_topics.asp?ForumID=39
I think it depends on how often you use it , how much you want to spend, a simple way to heat it would be 2 or 3 convector heaters on time clocks, how long is a piece of string?