We never leave the windows open if the caravan is empty. There is enough ventilation built in and in almost 40 years we have not had a problem.
I am not convinced by covers and at 77 I am not sure I am up to fitting them, they work for some people. As far as preventing leaks are concerned I suspect they simply mask the problem if you have one. If it does, the problem would possibly not show up until a subsequent owner had it though.
I think you have a very valid point. I am not a hero for Caravan roof covers, or Front covers.
. I store our caravan under cover in a barn and now get the van serviced every two years, in March normally.
So in theory if I put the van away in mid November and don't use it until March it has 4 months to dry out if it has a slight damp leak.
I know the van is dry as I check it myself. But if a someone was buying it in June it might have 6 months to dry out and I could unknowingly sell a van that has leak.
Two edged sword.
It all comes back to The Caravan Industry, should seal the caravans better.