Roy,
I have had onboard water tanks on my last 5 caravans, one of which was a Lunar 520/2. It is a matter of choice but i would not buy a caravan without an onboard tank.
We do a lot of winter caravanning, irrespective of weather conditions, just fill the tank and forget about the outside temperature.No danger of the aqauroll, or pipe to the aqauroll freezing. When i see some of the methods people use to stop their aquaroll from freezing, and then having to uncover it to fill it, then replace the cover i am glad i have an onboard tank.
Never ever tow your caravan with water in the tank.
You know when you will be leaving the site so you can use most or all of the water in the tank the day before leaving.
Most onboard tanks, like the one in our Swift Conqueror 655 hold about 45 litres, so one full aquaroll will fill the onboard tank.
The 520/2 tank is in the nearside rear corner in the base of the wardrobe. We found it worked well.The Shurflo pump gives good even water pressure.
Hamer