I have lived in rv's down to -25 degrees. Once it gets so cold, it doesn't matter much what package you have, it all freezes.... My son-in-law just got back from Williston ND in a well equipped motor home. Insulated skirting with heat and it froze solid,unattended while working.
In my opinion, some 4 season rigs make it harder to thaw out than a standard rig because everything is enclosed.
Now, if it is going to get much below 15-18 degrees, we winterize and use the porta potti and water jugs. With a good Catalytic heater, you can stay toasty warm and not worry about freezing up your rig.