We live in the Southeast so our winters are not as cold as most if not all of the people who have posted in this thread.
We love to camp in the winter time and have learned what works for us. We have been camping when its down in the teens at night and makes it in the thirties for the day time high.
Need to make sure you are able to keep the water supply warm enough as well keeping it liquid once its been used. You can purchase water hoses with electric heaters or add them to your current hoses. We also throw insulating blanket over the water supply pipe coming up out of the ground. We do not have them but I understand there are heaters for the dump tanks. They are intended to keep it above freezing.
We use ceramic heaters to supplement the furnace in the RV as I am not keen on running the RV furnace while I sleep.
We also realize that we need to bring a lot more clothes so we can bundle up when we do our outside activities.
Our TT does not stay comfortably warm when we camp during the winter. But we tend to make the best of the situation.
The best part for us is the campgrounds tend to be near empty.
Enjoy.