I do wish these arrangements were not called 'extended warranties' It is in my view misleading, warranties should be free. What you are purchasing is an insurance policy.
You should also be aware that as these are insurance policies, the people encouraging you to buy one are likely to be receiving commission for your business. In some industries the commission payments were a very substantial part of the salesman's employment package. If you really want to take one out it must be worth getting alternative quotes.
Now my comments do not relate specifically to caravans, but the same principals apply to all purchases:
In essence you need to assess the likely risk of the product failing in some way that is going to cost you to either put it right or replace it.
Also bear in mind that you also have certain protections under the Sale of Goods Act. This may already cover some types of failures, so why pay for something when your statutory rights give it to you for free? Though to be fair some failures would not be covered by SoGA. And as a product gets older SoGA is more difficult to apply, and the effort in pursuing a SoGA claim can become disheartening.
Even the policies have limitations, so it is vital that you read the policy terms and conditions.
In many cases -especially with electronic products (TV's etc) such policies are rarely necessary as they are so reliable, but it is a sad comment of the caravan industry their products have such a poor reputation for reliability.
Ultimately only you can decide if such policies are worth while. Some of these policies have I am sure been miss-sold rather like the PPI scandal!