You will need to try and get an engineers opinion, although if the chassis is bent I do not see how anyone can say when it happened. I would write to the dealer and say that the caravan has never been overloaded, hit a severe pothole or speed bump while in your possession and therefore the problem must have existed at the time you bought it.
The caravan is clearly defective, and under the CPA they must repair it or return your money. I doubt they will repair it, but that is the initial remedy. From a practical point of view you will have paid more than £10,000 for it I assume, this means it is over the small claims limit, which probably means you would need a solicitor. Alternatively if it can be repaired for under £10,000 you could sue for that.
If it was damaged by a pothole or road bump, then that is accidental damage and you might try an insurance claim. The insurer will arrange for an engineer to inspect it who would need to try and identify the cause of the problem. That mighty give you the engineers report.
A second line of attack if you paid any of the cost on a credit card, no matter how little, or used HP, is that you have similar rights against the finance companies involved, so let them know of the problem.
If you are a member of either of the clubs you could speak to their legal helpline. alternatively Citizens Advice are very helpful.
Hope this helps.