I think someone on the Forum made up a checklist, try a search.
It is not too complicated. Basically drain all water and leave taps open. Smear olive oil on the flap of the loo to stop it sticking. Do not use Vaseline. Remove all food and the gas cylinders if you can. Some people like to remove the cushions as they can be kept dry at home, and make the van less desirable to thieves. Try, if possible to keep blinds up and the handbrake off, again this will stop them all sticking. Obviously, leave the fridge door open, and the freezer compartment. I use the ice tray to hold the latter open. Remove any valuable items like TV's etc. Remove the leisure battery and give it a charge over the winter. A cover will protect the van but ensure it is clean and dry before you put one on. Some people are for and against them,. it is your choice. However, look at where it is being stored and if under trees then be aware of leaves and droppings that will mark the roof over time. While in store revolve the wheels regularly to stop flats developing, or get plastic plates that give a curved surface for the wheel. Some poeple like to leave either a low wattage heater on, or use a moisture extractor, but this all depends on where the storage is relevant to your house.
Yes, shops will suggest all sorts of 'goodies' Some work, some don't, but many peple just like to buy them to see, as it is part of their fun.
Of course many on this Forum will recommend the best answer is to keep using the van in winter. Personally, I don't but I am a 'chicken'.