Hi Helen,
First drain all the water from the water system, leave all taps in the central position in the ON position.
Drain the shower hose, remove the shower head and dry and store somewhere safe, leave the shower hose laying downwards on to the shower floor.
Drain all the water from the toilet system, both header tank and cassette, spray the toilet blade with silicone spray.
Empty, defrost and dry the fridge, leave the door slightly open to stop mould forming.
Either take all the soft furnishings indoors or at least move them away from the sides of the van to allow air circulation.
Leave all cupboards open and empty.
Remove the battery, take it home and keep it fully charged over the winter with monthly top up charging.
Disconnect the gas cylinder and put the end of the hose, or regulator, depending which type you have, bulkhead or cylinder mounted, in a cloth bag to stop over wintering insects making a home.
Put a few mothballs in the gas locker to deter mice, same inside the floor level cupboards.
If possible raise the van on axle stands and lower the corner steadies to make is stable, then release the handbrake to stop it sticking in the ON position.
If you cannot raise on stands, chock the wheels and release the handbrake.
I am sure others will add to this!!