The real problems I found in the past couple of years is with temps around 0 to 2 during the day and sub temps at night..this doesn't appear to let it warmup so things just freeze and continue to do so.
This year I am just completing my new water storage unit...it has a a 2000 litre water storage tank with 1000w titanium tube heater and digi-stat which is set to 5 degs add to this a 45w tube/boat heat with a frost-stat for the membrane as a back-up. The whole set-up is housed in a cabinate with a good air-space all round then 20cm's of polystyreen sheets covered with double sided silver foiled bubble wrap. The idea is to make the water once a week the water will then be heated and should stay at a reasonable temp without needing too much extra heating as new water is not being added to often.
Van is filled on monday when cold enough, remaining water in van tank is pumped back into holding tank using a boat sump pump in van tank at the end of day...small amount of alcohol (15% +/-) is pumped through the system to prevent freezing...job done.
Obviously this is all a non-event if it gets that cold to prevent work...in that event...I'm recording loads of my fav progs on tv to pass the time
& to stop the cabin fever again!!...
What are you doing to prevent the freeze hurting you?
Cheers
Dave.