Getting cold isn't it. Finished mine today just need to cover her up.
As for the fresh water system just like doing the water system in an RV. Turn on your pump and open the faucets until the tank is empty, no need to open the bottom of the tank. Turn the pump off close the faucets. Then put some non-toxic antifreeze in the tank and pump that through the system. My Kingfish has a water heater with a 6 gallon cap., two sinks and a deck shower that I installed. I usually put five or six gallons of -50 degree non-toxic antifreeze into the tank and then turn the fresh water pump on and turn on each outlet in turn until I get a good stream of anti-freeze. I know there are different ideas about quanity and formulas to figure quanity but for my boat the 5 to 6 gallons works well. Also be sure to put some down your fishbox drains etc. if they have low spots in in the drain hoses. I have a shower drain in the head with a pump too that needs to be done and if your bildges don't drain completely, depending on how you store your boat, put some in there too. Maybe overkill but it has worked well for me.