Hi oscar10, have Autotrail told you this directly after speaking to their service dept or is this a statement relayed via the dealer?
Flexing is common and is necessary but to what degree is any layman's guess.
When we were looking to replace our PVC we found both the Imala & Tribute had very similar characteristics in terms of cubicle flexing and found that the Tracker less so, I believe this is down to both having separate bonded wall & ceiling panels allowing them to flex independently whilst Tracker, Apache & Frontier seem to have a moulded pod which moves as a whole.