I must point out that just becasue some (not all) ABS panes may have cracked, it does not prove that all ABS panels are not fit for purpose.
It may still be the case there is a design or manufacturing fault which does not support the ABS panel correctly that allows the panel to be stressed beyond its yield points, so the crack in the panel is a symptom rather than the the panel itself being faulty .
But as age increases there is an increasing possibility the caravan may be exposed to excessive mechanical wear and tear or indeed other conditions that might precipitate the crack, and of course that increases the difficulty for customers to prove they have not caused excess wear and tear.
The fact is it needs a detailed independent expert engineering study to verify what the cause is, or if the manufacturers design specifications are inadequate. To investigate this possibility you would have to know what criteria the manufacturer actually specifies as the expected use case, and then if that use case is a gross under estimate of realistic conditions. This would be very very expensive to obtain, especially for a single user. - especially as some examples of the same model of caravan may remain crack free which would have to accepted as evidence that the design is not always flawed.
I suspect the manufacturer knows their products are not really adequately durable for British roads , but becasue they are bound by contract to the end-user they can hide behind the limitations of their warranty T&C's, and save time and money by ignoring out of warranty issues.
I believe the only way to bring real pressure to bear on the manufacturers would be to raise a class action.