I use a caravan cover during the winter months, it keeps it protected, however I usually get a bit of green on the roof where the condensation has collected and it can be a bit difficult to remove.
Before I put my cover on last winter I sprayed on the roof some Fenwicks overwintering protection, when I took the cover off this spring I had the usual green stuff on the roof but this time it could be simply washed off with some caravan cleaner, it isn't a permanent application and it is totally removed with caravan cleaner.
If you don't use a cover during winter this product would be ideal to make it easy to keep the winter grime easy to clean off when you come to use your van.

https://fenwickscaravan.com/product/overwintering/