Usually, more often than not, there is a little thing at the bottom of your contract that says "A NON-REFUNDABLE DEPOSIT of $X is collected to hold the vehicle and cover the cost of PDI, etc."
What is the situation? Why don't you want the RV anymore? Is if something that the business did?
How long has the business been sitting on the deposit? If it was weeks, I wouldn't expect my money back. They lost money by not having it available to sell.
Days? I would try speaking to the manager and explaining the situation. You did sign a contract, and really you can't get upset if they follow thru on it, unless it doesn't say it was a non-refundable deposit.