When you are expecting freezing weather, fill up your fresh water tank, disconnect and DRAIN your fresh water supply hose, sit back and relax. Since you are dealing with a used unit, I would crawl under the under belly and make sure all holes are stopped up, not allow any outside air to freely penetrate the under belly.
Also, if you are new to RV life, do not leave your black water tank drain open! Let it fill up at least 3/4 full before dumping. Lots of advice in other threads but bottom line, you need the full tank water to move the solids. Educate yourself on subject.
F250 HD 7.3 Diesel :)
31 Ft RLS Designer 5th Wheel :D