Check the internet first by looking for units similar to the ones you are considering for ACTUAL sales price, NOT the asking price. In their listings, eBay has "completed" auction prices and the prices in green are the ones that sold and for how much. Once you find an approximate price that the units are selling for it should give you a place to start. Don't put much faith in NADA estimates or other book values. Look for actual sale prices.
The more things you can stack in your favor the better. Cash sales have the best chance of getting accepted for the lowest price. Don't be afraid to offer a ridiculously low price because he is probably offering the coach at a ridiculously high price.
If you have to rely on the dealer to find you financing, you are in the worst position to negotiate for a lower price.
If you have decided on a certain model, gather information about all of the other units that are for sale so that the dealer knows that if you don't buy his, you can go somewhere else.
Know exactly what you are looking for before you go to the dealer. If they try to get you to change your mind, accept the changes only with a huge reduction in price. If the unit looks like it has been on the lot for awhile, make your offer very low. Don't worry about the dealer making money. USUALLY they won't agree to any contract at a loss.
Don't be afraid to just walk away.