The CRA does not say anything about having a product serviced, it's the manufacturers warranty that makes servicing a critical part of their terms and conditions, and it's their prerogative set the conditions for that. But the Manufacturer's warranty is an entirely separate process to the CRA and it cannot be used to manage or control the progress or outcome of a CRA claim.
As you quite rightly point out in respect of an end panel, apart from servicing the joint seals for water ingress, there is no service activity that would actively identify the fault before the crack occurred, or prevent or protect the panel against cracking. If a service had been carried out its possible the fault might have been identified earlier.
What I can't tell you is if the fact you have had water ingress through a seal, which should have been picked up during a dealer service, If that which has caused internal wood to rot, whether the reduction in the support given by those rotting wooden parts has caused extra stress in the end panel resulting in the crack.
Given vehicles like cars and lorries which do much higher mileages suffering the rigours of the UK's collection of potholes etc, and yet they manage in the main to be leak free. The methods of creating strong durable panel joints are more well known and understood, and I think customers should reasonably expect caravan panel joint seals should have similar reliability. What is a customer expected to do to prevent a panel from cracking?