RV dealers are regulated by the same agency that regulates automobile sales, Usually the states Department of Motor Vehicles.
This "Deal" where the price was quoted, the deposit made and then the price inflated, Has an aroma about it that is not at all pleasant. (Stinks in short) and your best options (Since you decided to accept the trailer at the higher price) are as follows:
In addition to posting, blogging and otherwise venting publicly (Which you are doing):
1: Make a full report to the proper DMV and let one of their people go in and make a deal like yours (This should frost them a bit)
2: Consult with a legal professional (Lawyer) however given the amount of the inflation ... that might not be worth it.. (you will have to decide)
To my way of thinking when the contract is signed and the deposit delivered, that is the way it should be.. That is the price it should be, that is the financing it should be. But alas, in the automotive sales business.. Your story is all too common.