This thread brings back lots of memories...some not so great.
I chased heavy construction, going from one boom job to another. Rather than be separated most of the year, my wife and kids opted to travel with us...so we lived in travel trailers for many years.
We spent one winter in Pocatello, Idaho at -35, and two winters in Big Piney, Wyoming...which got down to about -50 at night for a couple of weeks. It got hella cold in the RV...things would frost in closets if the doors were left closed, the door would ice over and you had to use a hair dryer to thaw it out in the morning.
And you usually left just a trickle of water running to keep the water lines in the trailer from freezing.
BUT, we survived all that and more.
You definitely want to skirt the RV well with sturdy material. Also, GOOD heat tape for your water supply lines, your sewer drain line, and even your holding tanks if they're exposed. I used "heat trace tape" from one of the pipeline jobs. IIRC, 5 watts/foot, self regulating...You adjusted how "hot" it got by the number of wraps per foot you used.
For a fifth wheel, not only do you want to skirt the body of the trailer, you have to skirt the gooseneck portion as well. Since that's often used as storage, be sure to leave a sturdy door in it that you can close and even lock.