Adam,
We got stuck in a surprise South Dakota blizzard near Sturgis one year in our '04 Monterey. I-90 was closed for a couple days. The wind blew up to about 50 mph. An awning blew off the next rig and a few picnic table rolled by. The temperature dropped to 15 below, yes, below zero. We kept the slides in for the toppers and the only heat was the HydroHot. It did fine! We stayed warm and comfortable.
I did begin worrying about what would have happened if the HydroHot broke. Over time I made some small changes to the electrical system and kept a few cheap electric heaters in the bay. I never had to use them.
20 degrees is nothing for a Beaver. As Steve said, your hoses will freeze. Fill the fresh tank, empty the waste tanks, close the valves and drain the hoses.