I have a Chateau 31E class C, and here is my experience camping in very cold weather. It wasn't -20F, but it was close to zero.
The furnace went through a lot of propane. Had to refill every couple of days. If you have shore power, then some electric ceramic heaters are helpful to reduce propane use.
I was not winterized, figuring that if the coach was kept at a comfortable temperature for humans then the plumbing would not freeze, and I had no problems with the interior plumbing. The external drain line for the freshwater tank froze, so de-winterizing after the trip was a huge pain.
I have heated holding tanks inside a compartment, so they were fine. However, the drain piping downstream of the tanks froze so I was unable to dump the tanks. Putting some anti-freeze in the tanks might prevent this, but I'm not positive.
The RV got covered with road salt during the trip, and I couldn't wash it well until the weather warmed up, and this resulted in a lot of corrosion under the chassis, corroded compartment door locks, etc.
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