I was thinking about the same thing a while ago (also for RA23). I have not looked into it much further, but AFAIK some European cars have electric interior heating (Saab, Volvo, BMW, Audi) that only works until the water temp is up. I guess you'll need some extra battery capacity though, and a bigger alternator when you want to run it all the time.
I think they call it auxillary heaters.
I guess this won't do?:
http://www.safetycentral.com/min12volcarh.html
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