Part of what you are fighting is that a private seller is often upside down on their RV and hoping to get what they owe. If it's a dealer then you are dealing with their markup. As azdryheat said you need to do your research and make your offer, and be ready to walk away.
Spend some time checking RV trader and other sites to get an idea of asking prices. Sites like crankyape.com and pplmotorhomes.com are good ones to use.
Beyond the price you want to make sure you don't get a lemon. If you know how to do an inspection you can find Pre-Delivery inspections all over the internet just search for 'rv pdi' if you don't then you can hire an inspector.