Perhaps the manufacturers did not do enough testing and research into the suitability of ABS in this particular application. That is not indicative of ABS being inherently wrong.
It however could be inherently wrong in this application?
Considering various factors inherent in this application, including but not limited to: -
Structural flexing of caravans both through towing and cyclic thermal exposure,
UV exposure,
Lack of control of consistent production build processes,
Its crack propagation characteristic,
The methods of fixing employed.
Where reliance is placed on it having to retain the product's watertight integrity.
IMO, its use in caravans should never have been for items requiring integrity, either structural or for water tightness.
It being however well suited for "finisher" type roles, where an underlying structure does all the integrity tasks.
Just as it is extensively used in the automotive vehicle industry, the cosmetic bits clipped on an underlying watertight working structure.