Craig, you do NOT have to constantly trickle charge batteries during the winter.
It is better, if yo ucan, to remove them from the van, as it makes it easier to look after them, but all they need is to be fully charged, then check them at about 2 monthly intervals and rechrge them. Leisure batteries WILL discharge over time, and its the possibility of it dropping below its usable low limit you want to avoid, which for a leisure battery is about 10.5 volts.