ruthiebaby88 we had a pop-up camper for 10 years and loved it. But now want hard sided and a few improvements over the pop-up.
That said, we winter camped in our pop-up for a 10 day trip to Alaska, and a 10 day trip back from Alaska, both in the winter!
We took water precautions (transferred 5 and 1 gallon containers from camper to truck cab) and had the fresh water system winterized. No grey water tank, just drain into outside jug that could freeze no problem and thaw later overnight. Black water was a porti-pottie with it's fresh water consisting of -40 degree windshield washer fluid. Yes the black would freeze, but thaw well enough to empty after all night with the furnace going. During the day we would have the furnace off.
The pop-up window plastic CRACKED at -30, but was quite flexible at -5. You live and learn. Read about our return camping trip/adventure here...
http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/27339362.cfm