will keep heat in like it does when you are in bed
There will be no heat in the tank. The heat will be in the van & the duvet will prevent any getting to the tank.
However, it's unlikely the heat would penetrate the tank really anyway.
That's what I like about the immersion heater, you have a tank full of hot water which acts as a 400ltr radiator- two birds, one stone an' all that.