Morganic,
The most impoetant thing to do is ensure you drain down all the water from every item that uses it, taps, shower, water heater, toilet,
If your van has water drain bungs underneath, it should tell you in the handbook, make sure you drain water from here too.
It is advisable to remove the shower head as water is sometimes difficult to get out and if it freezes, it will split the head.Leave all the taps in the open position.
Remove the battery and take it home, keep it fully charged in a cool but frost free place.
After you have drained the water, and the waste pipes are empty, place a loose bung of cloth in each outlet to prevent insects and vermin using the pipes as over wintering places.
Make sure the gas drop holes in the gas locker are mouse proof!! with wire mesh over them.
Either take all the upholstery home or stand it on end in the body of the van to allow air to circulate around the van.
Raise the van on axle stands to take the weight off the wheels.
Give the outside of the van a good wash before fitting the cover.
Disconnect the gas pigtail from the gas cylinder and put the loose end in a fabric bag tied around the hose to stop spiders or other small insects getting inside the pipe.
Probably something I have missed, but thats a good start.