We full-timed for 18 months in a Casita
SD17 including a cold December spent in an RV park in Utah. Here's our experience.
You can stay warm enough in the trailer, that's not the issue. Freezing the fluid lines is the problem. The holding tanks we never had issues with. The issue is the waste pipe and the grey pipe, and their valves that can freeze.
We lined the windows
with Reflectix and that was a tremendous help. We ended up using the Reflectix in the summer too, to block strong sunlight.
In our experience, pipes started freezing when the temperature went below 25 degrees and stayed there for any length of time. We froze our pipes solid more than once and thawed them with a hair dryer. Our Casita
held up fine having the black and grey lines and valves freezing solid. They worked after they thawed, no pipes or joints cracked.
The sewer drain pipe running from your RV to the sewer also can freeze as well as your city water supply line.
We skirted our trailer with concrete blankets and wrapped our city water supply line with heat tape and insulation. When we dumped we drained the sewer line completely. Have a straight, gentle incline between the sewer and the sewer valve is important. With those precautions our Casita
warmly housed us down to daytime single digit temperatures.
As I understand it, 4 season trailers fiberglass insulate their lines and heat their holding tanks. Sure would make it easier, but you can do winter in a Casita. It's not easy, but you can do it. The other full-timers in the RV park showed us how. They saved us!