In the past month I've met people full-timing as volunteers and COE gate attendants in north Texas for the winter in:
Airstream
Rockwood
Flagstaff
Puma
Open Range
Bullet
Outback
Grand Designs
Denali
brand trailers - some new, some up to 10 years old.
You have to know where you will be and what the limitations of the trailer are.
Almost any trailer with a fully-enclosed underbelly with heat pads under the tanks and the propane heater vented into the underbelly will be fine in winters down into the teens.
Only if you are going to be in temps down near 0 often do you have to ensure you get a fully four season rated trailer.
But even then, you need things like heat tape for the fresh water line, and provisions for the sewer hose.
The reality is many of us cannot afford the trailer we would wish, so we have to get the best we can and make it work.