I did some research and thought carefully about the position before writing on this case.
The automatic right of rejection under the CPA is only for faulty goods, except for mail order and one or two other things, but certainly does not apply to caravans bought from a dealer.
To reject within thirty days you have to show that the goods are faulty, but this might be difficult here as we simply do not know if the repair before taking possession was successful and this is residual damp that will dry out or is it a fault and there is still water ingress. If that repair was successful then the caravan is not faulty.
We also do not know if the thirty days to reject have expired, in which case the only recourse is an exchange which is difficult with a second hand caravan or a repair.
This one is not straightforward, and this is why I suggested legal advice.