Your dealer is telling lies.
No caravan is designed to let water IN, at least not intentionally.
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From the Which website:
Repair or replace
If you are outside the 30-day right to reject, you have to give the retailer one opportunity to repair or replace any goods or digital content which are of unsatisfactory quality, unfit for purpose or not as described.
You can state your preference, but the retailer can normally choose whichever would be cheapest or easier for it to do.
If the attempt at a repair or replacement is unsuccessful, you can then claim a refund or a price reduction if you wish to keep the product.
You're entitled to a full or partial refund instead of a repair or replacement if any of the following are true:
the cost of the repair or replacement is disproportionate to the value of the goods or digital content
a repair or replacement is impossible
a repair or replacement would cause you significant inconvenience
the repair would take an unreasonably long amount of time.
If a repair or replacement is not possible, or the attempt at repair fails, or the first replacement also turns out to be defective, you have a further right to receive a refund of up to 100% of the price you paid, or to reject the goods for a full refund.
If you don't want a refund and still want your product repaired or replaced, you have the right to request that the retailer makes further attempts at a repair or replacement.
Use our step-by-step guide if you want to ask a retailer to repair or replace something you've bought that subsequently develops a fault.
The first six months
If you discover the fault within the first six months of having the product, it is presumed to have been there since the time you took ownership of it - unless the retailer can prove otherwise.
During this time, it's up to the retailer to prove that the fault wasn't there when you bought it - it's not up to you to prove that it was.
If an attempt at repair or replacement has failed, you have the right to reject the goods for a full refund, or price reduction if you wish to keep the product.
The retailer can't make any deductions from your refund in the first six months following an unsuccessful attempt at repair or replacement.
The only exception to this rule is motor vehicles, where the retailer may make a reasonable reduction for the use you've already had of the vehicle after the first 30 days.
If you'd prefer to keep the goods in question, you can request an appropriate price reduction.
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Make sure you put everything in writing , keeping a copy for yourself and insist that the dealer puts everything in writing.
If you deliver anything by hand, take a photo at the time of handover with a time and date stamp. Otherwise, recorded delivery for all correspondence.