My general rule of thumb is to worry when the temp goes to 26ish at night and doesn't go above 50 or so during the day.
This has worked for the last 6 years I've owned my "C".
I also blow the lines out with air instead of filling them with the pink stuff. I made an adapter to connect my air compressor to my water inlet, regulated to 25psi. I open the traps and leave the air run for about 45 minutes to work out all possible moisture. Pink stuff goes into the sink traps and tanks, main water tank and water heater drained and traps left open. Use rubberbands to hold an old sock end onto the outlet. This prevents bugs from coming in. If stored outside, cover the wheels and hang newspaper so it hangs on the window side of the curtain rods. Block out all possible sunlight. I've installed trickle chargers for the main and coach batteries, and they run on a timer so only at night (cheaper electricity). The alternative would be to remove the batteriesand store them inside for the winter. Be advised if you bring them in... They CANNOT be stored on concrete flooring. For whatever reason, concrete will kill the battery. You need to elevate them several inches. you will probably need to recharge them in the springtime.
I store mine in this manner so I can use it at will anytime during the winter. "Opening" it up for use is easy and so is storing it again.