Now that the cold north winds are blowing, this might help for to keep the inside of the van moisture free. You can get oil filled tubular green house heaters of varying sizes i.e. 1,2,3,4,5,6 feet long and corresponding wattage on Ebay for £15 and plug in one of those time control adaptors so that it comes on for what ever time and lenght you want. They are low power so not expensive to run. But remember to run them flat i.e. horizontal and not vertical .