You are camping with your girlfriend, and you don't know how to keep her warm at night?

Well, anyways, lets assume that putting hot rocks from a fire in your tent is a good idea. (I will leave that one up to you.) It is more efficient to just use the rocks. They are the source of heat. The more "things" between you and source, the less you get. Energy is always lost in the system.

Personally, I have never had a problem keeping my wife warm while camping.