It’s important to face the reality here: most RV dealers are in the business to make as much money off of each sale as they possibly can.
There are four main parts of the deal that can impact how much you spend, and it's worth understanding each of them before negotiating:
Down payment
Purchase price
The purchase price is the most obvious part of the deal, but it's only one of the four levers that finance departments will use to make money off of you. Some dealers will try to make up for a low purchase price by offering you less for your trade-in, or by increasing the interest rate on your loan.
Some dealers set fixed purchase prices and don't negotiate. This is less common, but more typical with dealers who sell in large volume, as they make their profit by selling in bulk.
If you're buying from a dealer who does negotiate, it's worth doing your homework to understand what a fair price is for the RV you're looking at. Sites like NADA Guides and RV Trader can help.
Down payment
The down payment is the amount of money you pay upfront when you buy an RV. The more you put down, the less you'll have to finance, which can save you money in the long run. Dealers typically like to see at least 10% down, but the more you can put down, the better.
I'll cover the other two pieces of the deal on their own pages.