Flat battery

I keep our caravan on the drive and i did not leave it hooked up to the mains after we last used it,  as it has a solar panel on the roof, its a coachman vip460,  and i have been checking the van regulary, and the battery was good,  after this last lot of snow  i checked the van yesterday and the control panel was blank nothing worked, so put multi-meter on battery and it read 3.6v, plugged van into mains and left overnight, read voltage again and just the same, so i removed the fuse from solar panels, disconnected the battery and put in the garage and hooked up a charger its now reading 13. 5v  so will leave it charging and see if its holding a charge otherwise its a new battery,  but will leave it till weather warms up a bit as not using the van. good job its not stored miles away.