Hello Mark,
Warranties and guarantees are not well understood:
A warranty is technically is rather lake an MOT where it is a statement that at the time of sale to the original retail purchaser that the goods are of merchantable quality and free from design, material, and workmanship defects. The only person who can issue such a warranty is the seller. The sale of goods act requires the seller to offer such a warranty. It exists through the contract of sale, and hence is only compliant on the parties to the contract.
The consequence of this is that warranty work is the legal responsibility of the seller - no one else. And only the original purchaser has any legal right to it. Six years is the effective maximum time that the courts will enforce liabilities, but consideration is given to normal wear and tear and reasonable life expectancies of the product.
As you have the caravan second hand, you have no legal right to the original warranty.
In addition most manufactures do offer the original purchaser a 'Manufacturers Guarantee' This is always in addition to the legal warranty and cannot supplant it in any way. It is not a legal requirement and as such it can be tied up with many clauses, such as only being available to the original purchaser. Some manufactures will transfer it to subsequent owner, but they are not obliged to, and they can charge to transfer it, or make it subject to certain conditions such as subject to an survey.
Sadly I do not think you have any claim, against the manufacturer or the original retail seller. - However, as RogerL suggests if you were sold the van by a trader as having the manufacturers grantee, then that is misrepresentation and contravenes the Trades Descriptions Act.
Equally if the van was sold to you by a trader, but was not specifically 'sold as seen' or that water ingress was specifically excluded for the seller liability, then you may have warranty claim under the Sale of Goods Act against your seller.
Second hand sales can be difficult and I suggest that before taking any action seek professional legal advice.