Hello Peter,
I fully understand your concerns, and it always annoys me to read of caravanners who have suffer water seal failure ingress, which simply should not happen on modern caravans.
I must first say that under the circumstances you describe which are not entirely clear cut, if you wanted to pursue the matter under SoGA, It would be wise to seek professional legal advice.
SoGA has three important time limits, the first is 6 months from point of sale (PoS) and the second is the normal life expectancy of a product, and the third is the 6 year limit on SoGA claims.
If a product fails within the initial 6 months (and it has not exceeded its normal life expectancy e.g. perishable or consumable goods) then SoGA assumes the product was faulty from new. The seller is obliged offer either a full refund of all costs or a replacement.
Beyond 6 months the onus is on the customer to prove any fault was present at PoS.
As the caravan is 8 months old the 6 month time limit has expired.
In essence you would need to prove by the balance of probabilities the fault was present at the Point of sale. That I suspect will be quite difficult.
You need to consider? in which ways does the product fail the SoGA test:-
Is it a faulty design - unlikely as other vans of the same design may be fully dry.
Is it faulty materials or workmanship? - this is likely, but without a careful inspection of the product its difficult to know for sure.
Is it fit for purpose? - Sometimes you have to look at the question from a different perspective such as; Did the product fail to work completely?, and here the answer is that it has partially failed.
Is it durable and of merchantable quality? - this also dependent on careful inspection and comparison with what a normal person would expect.
In all of this the seller may suggest the caravan has been abused or mistreated which you would have to prove otherwise.
Being pragmatic about it, A competent repair can be a perfectly acceptable solution. The key test will be if the caravans has a lower value than the same caravan that has not suffered the fault.