If the battery is not required to power an alarm, remove it , take it home and keep it fully charged over the winter.
Drain all water from the various place in the van, water heater, toilet flush , internal tank if fitted.
Leave all taps open (for single lever taps leave in central position open) and remove the shower head
Remove any filter that may be fitted.
Clean and dry the fridge and leave the door and freezer flap open.
Give the toilet blade a squirt of silicone lubricant and leave the blade open.
Either take home (if room to store) the soft furnishings, or move the cushions away from the van walls to allow full circulation of air.
Make sure all windows and roof vents are shut and locked.
Remove ALL foodstuffs.
Make sure tyres are at their proper pressure, wheels chocked and handbrake OFF.