Does lack of use increase the speed of rot?

Regular driving can give better air circulation which helps prevent rot. But a lot also depends where you store them. Somewhere with a well drained surface that gets some warmth and good air circulation is going to be a lot better than somewhere like my last driveway - compacted earth under gravel with hedges on three sides. Cars were always damp there.

Cover don't help, they prevent air circulation.

I personally think it's BS, look at these old cars that turn up with low miles etc, they have obviously been unused, that is how they've survived, most of the cars that are from the same ilk are long gone though use and exposure to the elements, but obviously this all depends on storage.

But it likely had rot already which was unnoticed which made it get worse. And the fact it would have in rain day in day out can't help!

Most of the issues I've had repaired on my camper project could be down to lack of use. One of the suspension top bearings seized causing the spring to wind and move. The rear brakes seized. Last year I hardly used it, but this year I've kept it taxed a lot more and I've been making an effort to give it a run out even when I'm not using it for weekends away.

My advice is to use a glass-area cover, and park it where it's in line with the sun most of the day. The upper cover will help protect the interior from UV damage but allow prevention from damp/mould, and the regular sunlight/warmth will keep the rust away. Exterior plastics can suffer, some worse than others (tail light lenses are terrible), there are treatments to help and it's a small price to pay compared to a rotten shell.

This is coming from someone who had a solid 27yo car, parked it in a deep bushy corner where it never got a minute of sunlight, and saw it dissolve into a cornflake-on-wheels in just two years.

I've had my golf stored for the last 9 months now in a garage where the sun gets on the door for most of the day and warms it up to unbelievably oven like temperatures, I'd say it's no different now to the day I parked it in there bar a flat battery and some dust (oh and a dent and some scratches where a wheel fell onto it baaaaaaarrrghhhh!!), I check it every two weeks or so just to make sure no mould etc is forming, I leave windows and boot open for two weeks, then shut them for two weeks as well as move it out to check it all in the day light if time permits. Once it's over at my house it'll be getting new brakes, cables, oil etc anyway along with a new turbo.

As others have stated a car which moves will air out, the vibrations, wind from driving and heat from the engine and brakes/tyres etc normally play a small part in drying away quite a lot of water, if a car is sat this doesn't happen and water is allowed to accumulate.

My Triumph Toledo was much the same when left outside near bushes for 4 months, whether that was my own perception of it looking sad or the fact it was actually getting worse.

Personally if I ever have to leave my golf outside, I'd rather sell it to someone who has shelter. I understand its a van but are there carcoons big enough?

It's a campervan that comes out half a dozen times a year for weekends away.

Never thought about like that, it always ends up on a campsite field. That's got to be a bad environment for encouraging rust to get worse.

I had a 8 year old golf mk2 years ago from by the sea and it was rotten as a peach underneath .Looked fine on the outside but it was so rusty you could see through the floor and the seat was sinking !