It is easy to blame the first time purchaser for the overlooked issues. You went into this purchase thinking that the dealer would quickly do the right thing and now you experienced reality. Don't blame yourself on missing these things, you probably had faith in your fellow man.
What to do? The dealer has you over the barrel and knows that you will never be a customer again, so he couldn't care less. There is little chance that the deal can be reversed, but just push the issue till he can't stand it. Contact the factory and tell them that the trailer is not fit for delivery.