If we were in snow again, we would probably bungee a tarp around the hitch. We have a vinyl cover but it would likely freeze and crack.
I didn't think Oregon winter was really severe at the coast. I take extra steps in extreme cold that you can try if needed.
The truck cab is your evil cold air zone. I bought a cheap tension curtain rod at Wal-Mart that reaches across the rig at the ceiling behind the cab seats. I put it up and hang thin silver Mylar emergency blankets like drapes. I fold them over the rod and clip with clothes pins then fold where they meet in the middle and clip. I clip to the privacy curtain tracks on the side angled walls. I then tuck the Mylar against the side walls along the floor and put shower shoes or something on there to hold them in place. We give up the cab space in extreme cold but can open the center of the mylar " curtains" if we need something up there. You can set the rod lower so the tv is visible.
For a quick cold blockade I just hang the Mylar waist high clipping to the privacy curtain tracks (no tension rod). You can lean over it and reach into the cab. There is a huge temperature difference between the house air and the enclosed space behind the Mylar.
We have a small space heater we use with our A.C. heat strip when we have electric connections and we run the propane heater when we don't. We try really hard to not camp below 40 or so without electric but we've done it a few times. We consider 50 good sleeping weather so we may tolerate colder temps than others are happy with.
NOTE - remember to close your truck vents and your house fan vents. The truck has an " off" setting for the vents on the climate control knob. Turn this to off and close each dash vent louver for good measure. Double check the bathroom vent before bed. We often forget it and send wonderful heat up to the sky.
I recommend house shoes or slippers and an extra blanket on the bed. If you are running on electric heaters and it's too nippy to get out of bed, hit the propane heater for a quick warm up then go back to electric if you like. A lot of people avoid using propane since they are already paying for electric at the site but I get cranky when I'm cold so John is more than happy to kick the furnace on if needed. The cost of propane is a small price to pay to avoid being stuck in a small space with a cranky woman.
Enjoy your trip! We can't wait to get back to the Oregon coast. We spent part of one winter there (through January) and plan to be back to winter there in 2018.
Oh, we had moderate but wet weather for a good stretch there and bought a tiny little dehumidifier to help handle the massive amount of wet stuff we had in the rig - rain jackets, rain pants, boots, bath towels, kitchen towels, dishes, etc. Our windows fogged on the inside and we were perpetually damp. The little dehumidifier did the trick.