Just retrieved my Triton from its outdoor winter storage location. It has had a cover on, but has leaked at some point from the awning rail at the front. Also, when I popped up the top at home there was a mould area on the underside of the roof where the fabric bunches up. Cleaned off easily, but has left a stained roof panel. I have sealed up all the vents and stuck my dehumidifier in overnight, but no water captured yet, so any leakage must have been minimal (will probably increase as the temperature goes up with the sun on the van...). Unfortunately the front wall inner skin has become "textured"

Well report back from winter 2023\2024. After last year’s disaster where the Eriba of 1 year resembled a stilton factory, I am glad I stuck to my guns.
This year Eriba covered again, same as previous year but with a dehumidifier on auto with the hose going straight in to the sink and not a spot of mould anywhere, so this was a massive relief as you can imagine compared to last year, so if you get damp/mould this is definitely an easier way to eradicate it IMO

As said you can always pull moisture from the atmosphere in a van because the fixed ventilation ensures the moisture content is always the same as the air outside. You can reduce the risk by leaving the van as open as possible inside to encourage air circulation and ensure the van is in an open area and not in constant shade of buildings or trees.
Having had nine leisure vehicles stored in winter in five different places over 18 years I've never had a mould problem and never employed dehumidification.
There's just one oddity that I came across with our new Bailey last year. It has it's spare wheel inboard under the nearside lounge bench and. the wheel has a sort of strong hessian like cover over it. At the end of last winter our cover was covered in mould and many other owners reported the same but like us had no other mould issues. To me that suggests that something in the material reacts with moisture in the air to create mould and if a surface or material is inert mould will not happen. I suspect that I'll find further mould on that spare wheel cover when I re ennervate the van later this week.

I know mine is still leaking a bit somewhere along the top rail, so complete removal and replacement of the mastic bead is planned this year (it was last year, but the weather didn't cooperate and I don't have access to any covered space to work in). I have all the goo ready to go...

I have a small dehumidifier I pop in the van when I bring it home from storage - and that works well. But I do cover the ventilators - and it is warmer by then (I hope!)

I also open everything up on breezy, dry days - and that's probably the best thing!