At the temps you are talking about the unit should function properly. It takes well below freezing temps for long term to start affecting the cooling unit. You can close off most of the vents on the back outside but not all, leave at least one open especially if you might be running it on propane, if on electric it won't matter as much. We never closed off the upper/roof vents. You can put a 60W bulb in for extra protection if parked so you have power.
The most vulnerable item is your icemaker valve, which can freeze at temps just below freezing. We had ours break in the driveway and accidentally left the pump on...it drained the fresh water tank and floored the inside as well as ran down the driveway. We now don't use the icemaker because don't like tank water ice cubes. Also, shut off the water line to the icemaker valve just in case the line or valve should crack.