Expanding on that idea a little...

How 'bout using a Solar Shower to heat the tent? It'd take a little planning ahead, but 4 gallons of water at over 100F would have to do something useful in a small, enclosed volume (tent), wouldn't it?

Just fill it up, leave it out in the sun, then bring it in at night.

Edit: Or not. That's only 32 lbs of water. It'd be like having a small child with you. Unless it gets hotter than 100F. (The ad says 108F... wonder how hot would make it worth it?)