Geza, 

depending on where you camp.. 
I use an 120 volt electric banket, or 120volt ceramic space heater (safe, no fire hazard) when I have hook ups. 

when don't I carry a 12volt electric warming pad,  this + a 2nd aux battery will keep your daughter warm all or most of the night.  (example Ebay item number: 5878287962 
http://cgi.ebay.com/12-VOLT-FLEECE-PLAID-ELECTRIC-HEATED-CAR-BLANKET_W0QQitemZ5878287962QQcategoryZ73335QQssPageNameZWDVWQQrdZ1QQcmdZViewItem 

and when you don't need the 12vt blanket you still have the use of the 2nd battery, always a good idea. 

this is the simplest set_up w/o permament modifications to the body&interior.. 

though I still say go w/ the furnace.. but the costs of the furnce, installation, 2nd battery, and 2nd propane tank (extend a stay tank adapter)  will keep many frugal campers putting flower pots on thier stoves (BTDT) 

dan 

Geza Polony <gezapolony@SBCGLOBAL.NET>
Subject: Re: 

Al, it's largely for my kid, who freezes in the upstairs bunk. I'm ok with
the down cover, the wife, and, uh, the dog...mostly for my daughter who
complains about cold up there.

The radiant idea, whether with flower pot or cast iron pan or whatever,
doesn't solve the problem of unvented propane flames burning.

I'm well aware of the problem of asphixiation. This particular van, though,
isn't exactly air tight; in fact, with the top up it has quite a nice cross
ventilation even with the window closed. Well, if it did close...

So in fact, there's plenty of combustion air. Wifey's concern is mostly
about some dummy dropping a sock into the heater, like probably me.

I sort of like the idea of strapping it on somehow, then putting a cover of
some sort over it. Time to get out the thinking cap. Before I drop it in the
heater, anyway.