Make sure the van is parked where the panel/s get to see some sun, the longer duration the better and make sure the panels aren't covered in snow.
Saying that you don't want to be charging a lithium battery around freezing as that will degrade the battery, down to 3C and you're probably ok, if the leisure battery is an AGM or simple lead acid no low temperature charging problems.
I've got a lithium leisure battery so have turned off the solar panels and isolated the battery so it doesn't charge while driving the van and leave the battery fully charged for weeks/months which again can be detrimental to battery life on lithium types in particular.