One of the first things we did to our popup is trade in sleeping bag camping for real sheets and pillows. The easiest way to stay warm is by using flannel sheets and a down comforter.
I purchased the Pinzon Heavyweight Cotton Flannel sheet set, which has held up quite well.
I purchased the Pinzon Pyrenees Medium Warmth Down Comforter. My wife and I like it so much, we bring it into the house during the winter for use on our bed. We tried the Extra Warmth model first, but found it too warm since the popup camper has a heater.
I also recommend storing the comforter in a protective sack to keep it clean. We use the Kelty shown above.