Hello Chris,
Have you told the dealer about your problems? or are you assuming that they wont help. If they are a reputable dealer it is in their interests to keep customers happy.
With all secondhand purchases even from dealers the phrase Caveat emptor (Buyer beware) still holds true. In essesnce what this means is that as the buyer, it is up to you to ask all relevant questions about the item before you buy.
If you ask a question you should be able to rely on the answer as being accurate and honest, unfortunately with a private transaction it is often difficult to hold the seller to book, but at least with a trader you have some advantage, as they are bound by trades descriptions, and have a trading address.
Now this is important, because if you asked the trader about the condition of the caravan and they indicated it was dry or free from water ingress, then you may have a claim regardless of their rider in the guarantee. It is difficult to see how a caravan at age 10 Years could be dry yet at 10Y4M is wet.
You may also have some leverage if you told the dealer when you started looking at caravans you were new to caravan ownership and you asked the dealer to advise you. By doing so you are placing yourselves in his/her hands and are relying on them to use their expert knowledge to guide you.
As an expert, they should be aware of the increasing chance of water ingress problems with the age of the caravan, and they should have brought this to your attention so you could make an informed decision. If they failed to do this it might be construed as incompetence, or worse a deliberate act to conceal the truth (fraud).
Before deciding to take any further action, I strongly advise you to seek professional legal advice.
A letter from your solicitor may be enough to stir some action, but if it does go to court, the age of the caravan will certainly be major factor in any judgement as will the fairly well publicised issue of water ingress in older caravans, it could be argued you should have know or expected damp issues.