Condensation on roof lights

For the last 9 years I have had a 2003 Lunar Stellar 400, which I have kept covered in the winter with a Specialised Accessories cover, and have experienced no condensation problems.  Last spring I bought the current model of the same van and we used it several times during the year with no hint of problems.  In November I covered it for the winter with a new cover of the same make, and am now finding all the roof lights covered in condensation.  This is particularly heavy on the large Heki light.  (It runs down your arm when you mop it!)  There is no condensation on the roof itself nor on the walls.  I am confident there is no water ingress.  Can anyone suggest why this van is different and any solutions?

Inchberry