The problem here is that this is stamped on the contract and unless you did something at the time I feel you would be considered to have accepted this, although there is a chance it would be regarded as unfair in court. Unfortunately even proving what happened is potentially hazardous as it may be your word against theirs.
The real problem is what you do about it, as if you really want the caravan, any dispute could end with the dealer attempting to cancel the contract on the basis you are not willing to accept it, leaving you to challenge them in court. That would probably end up with you getting damages for breach of contract rather than the caravan, assuming you win. Frankly I feel the problems in pushing it are too risky for the sums involved, and you either bite the bullet and pay up or just cancel demanding your deposit back. Not knowing the dealer, I do not know if there is room for compromise though if you challenge them, and paying the extra and then raising a small claims action for this might work, but nothing is guaranteed to work.
If you have access to free legal advice I would take this.