Hiya, yep ive wrapped the tank a couple of times in the silver bubble wrap type insulation.
My van is a fiat scudo to give an idea of internal van size and since having the immersion ive not had to worry about things freezing. But then my pump, batteries and hoses have always been quite close to the tank as well. So when the first beast from the east came, my van and equipment was ok.
Ive now changed the van layout and weve had this mini beast hit over the wknd. My water in the tank on the friday was around 60 degrees ish by end of fridays work, maybe late 50's (didnt take a reading) but with not working over the wknd I didnt run the immersion over night and woke to a good helping of snow. We had about 2inchs and so dashed to van to check and all was well. By end of sat night the van temp near the back doors (furthest away from the tank) was showing around 5/6 degrees whilst outside was - 3. Then sunday late evening I had the immersion on for Monday's work, so alls been good with this layout.
But it might depend on what temp youve got the water inside the tank to how effectice the ambient temp will be and the size of ur van