Solar Panel Battery Maintenance Charge

hitchglitch

I know this subject has been done to death in the past but I cannot find what I am looking for using 'Search'.  I need to keep the 110 Ahr van battery topped up during storage and have just noticed that Halfords have a self-contained 6 W panel (made by Ring?) for about £40. It has a 12 volt plug so no need to connect in to the battery terminals. It is specifically sold as a maintenance charger and is the largest of the range.

My concern is that I will have to keep the 12 volt main switch on to charge using the socket outlet.  I need to check whether there is a fridge light but, that apart, are there any other permanent drains on the battery apart from the alarm/tracker? Will 6 Watts do the job during the winter?