Iain,
1. Drain all water from your hot and cold water systems, leave all taps in open position and remove shower head.
2. Empty the toilet flush water from the tank, empty and clean the toilet waste container, apply a spray of silicone to the rubber toilet flap and leave in the open position.
3.store all the soft furnishings on end in the van, if they cannot be stored at home to allow air circulation.
4. Leave all cupboard doors open, again for free air circulation.
5. Clean out the fridge and dry the inside, leave the freezer door open slightly and the main door open.
6. Remove battery and store at home in a dry, frost free place, preferably not on concrete, place on a piece of wood or thick cardboard and charge every couple of months to maintain it in good condition.
7. Either raise the van on axle stands to take the weight off the tyres, or move the van every couple of weeks to avoid flat spots affecting the tyres.
8. Disconnect the gas bottle and put the cap back on the bottle, put the regulator in a fabric bag to stop insects crawling into it.
9.If you dont want curtains fading, cut either cardboard templates of the windows, or something similar to keep the light out.
10. If you think it a good idea, place moisture absorbing crystals in the van (packs available at Woolworths).
11 Make sure all the windows and roof vents are securely closed.
12 Check your van regularly whilst in storage and especially if there is bad weather.
If I have missed anything, i am sure someone else will fill in the blanks.