Thank you for the history.
I had hoped your purchase may have been more recent, becasue you may have been able to claim against the supplier even though it might have been outside the dealers warranty period. Sadly I suspect attempting to claim for a miss sold vehicle under the Consumer Rights Act would be very difficult to prove under these circumstances.
I am acutely aware in situations like this, we only have one side of the story, but on the assumption you have accurately recalled the proceedings:-
You should be unhappy with the company who assessed the motorhome in 2019 who advised you a damp reading of 53% didn't need urgent attention. That's worrying, and represents less than professional sound advice.
The balance of probabilities suggests that your motor home will have deteriorated further since the initial 53% finding last year. What seems far less likely is the damage expanding by such a large margin to lift a £2500 estimate to £16000 over the same period.
Its worth pointing out that provide the repairs are done to a good standard, the motor home should still function very well.
Its my opinion the original assessment most likely underestimated the extent of the damage at that time, and if the work had started then, the costs would have escalated as the true extent of the damage was revealed. It is also seems likely this is a problem that has been ongoing for some considerable time, and might have been started before you purchased the motorhome.
In view of the projected costs there would be nothing lost if you were to ask if the dealer who sold the motorhome to you might be prepared to contribute to the cost of repairs. They might be encouraged to help if they understand you are aware of your statutory rights under the Consumer Rights Act 2015.
It would need a respected professional engineer to review the damage, and if they can conclude the damp must have been present, or the cause of the damp was present at the point of sale, then you might have a case under the CRA, but as its more than 6 months since you purchased it, you would have to provide the evidence to support your case, and be aware the longer you have a product the more likely it is to develop faults and of course it may have been damaged in use resulting in the water leakage.
Read up about the Consumer Rights Act (CRA) it might provide some respite.