If you have an enclosed underbelly on the trailer you may very well have heated tanks. Usually all it consists of is a duct leading to the underbelly area from the furnace ducting. You have to run the furnace to have it do any good.
Without that, there are several things you can do to camp in below freezing weather.
1. Use your fresh water tank onboard to supply water - if the tank is above the floor as many trailers have.
2. Remove your water hose from the campground pedestal after using it each time.
3. Open cupboards to let warm inside air circulate among the pipes inside the trailer.
4. Keep your gray tank valve open and let several faucets run or fast drip to keep the water flowing and prevent freezing.
5. Use bottled water for everything and forgo using the trailers water system altogether. (This is usually what we do when traveling South for the winter for the first couple of days. We carry a gallon of RV antifreeze to use for flushing the toilet and we dump all waste water into the toilet also.)
Hope this helps.
Barney
I am going to move this to the General RVing Issues forum. This forum is for DIY modifications and upgrades. Thank you for your post.