What's difficult or misleading about Model Year and Registration Date, or original purchase date in the case of a UK caravan.Odd? I don't think so. The clue is in the question.
Build date - should be answered with the date it was built
Purchase date - should be answered with the date you take possession,
Model year - should be answered with year the manufacturer has designed it for.
But I agree that these staggered artificial years can be misleading. But then why does the Tax year start on April 5th?
It's very convenient for industry, car and caravan, to change production during the summer shutdown - any insurer who can't cope with it for caravans is best avoided anyway.