I have had a PF van mount fitted for the last 3 years. All purification etc in van. Only had one problem. I always put an oil filled heater in van when below minus 3.
The one problem I had was last Jan, didn;t work for weeks, even with the heater on (was low), the pipe from pump froze but no problem after warming.
I find as long as water trickling into van it won't freeze. Mind you I live nr B'ham not North Scotland