Might be worth considering how much insurance would be compared to value of van? Possibly self insuring ie put £150 away a year that you might have spent otherwise.
Obviously your risk goes down longer you are "claim free" and you end up with pot to spend on newer caravan at some point?
Just another view to consider, although I appreciate if you use storage site you may require insurance for them to accept you
Google "caravan insurance comparison sites" and plenty will come up where you put details in once and any willing insurers will be displayed
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Many forum members use the club caravan insurers, but these are only cheaper if you are members of these clubs for others reasons. If all you use is their insurance offers then to any insurance premium, you must add the cost of your club's annual membership fee and so your insurance costs can work out a lot higher than is at first obvious.
We used to be with C&CC and their insurance and it took us a while to realise that we could get better insurance elsewhere for a lot less money. So it does pay to shop around.