I have never really understood why people lag there tanks
Mine has never froze! It's been in temperatures of -10.
How can the water in the tank freeze? The large volume of water in the tank is enough to stop it freezing.
All you need to watch is your pump, varistream, hose reel, and all pipes.
Don't worry about the tank but do worry about all the parts and components you have in the van/car/trailer