Remember that the cubic feet of air that needs to be heated is extremely small so a high output heater is not needed. A 5 gallon tank of propane should have for at least 2 weeks of cooking and heating and to power the fridge.
There are two drawbacks to popups, the amount of storage space and the size of the fridge and holding tanks. If you can live with those aspects then winter camping is not going to be a problem.
Even a hard side camper is not a great winter use RV. The walls are much too thin to provide effective insulation. In weather down into the teens for several days and nights in a row our water pump inside our hard side camper will freeze up.