The winter covers are an insulated cover which goes over the fridge vents, not all vans are supplied with them, mine wasn't. I only turn the fridge on before I head off and clear it out and off once I get home. Covering the fridge vents when not in use helps keep the cold damp air out of the van and means that any damp traps or dehumidifiers you use don't have to work as hard. While I agree with others that starting the van for a period and taking it out for the day or a short drive is the best thing for it, I have been guilty of neglect this past three weeks as I have been away and haven't done any of them, I will resolve that this week.
If it is the complete cover you are thinking of which goes over the van, I wouldn't bother as they mean that you will really neglect the van, use it and come spring give it a good top to bottom clean, that way you will know it inside out and can sort out any imperfections which you find.