I don't see where knowing the dealer cost helps much, since there is no well-established margin that dealers use to cover their expenses and make a profit. They probably pay somewhere around 60%-65% of the MSRP, if the figures you are looking at are an actual Manufacturer price. Since Rvs don't have a federally mandated "window sticker", the price list you see at the dealer lot may be fabricated by the dealer and include add-ons and fees of his own choice.
Experience has shown that RV dealers typically offer to deal at something like 12-14% below the real MSRP, often terming that price a "RV show special" or "today only" discount price. However, you may be able to negotiate discounts as deep as 20-30%, depending on a host of factors, e.g. the dealer's sales volume that month, seasonal demands, manufacturer incentives at bigger RV shows, etc. Smaller dealers sometimes have cash flow problems that push them to make deals simply to get some money in the bank or reduce their floor plan costs. It's a highly variable situation and not at all like the car dealer market.
Make sure you compare based on out-the-door pricing, with all delivery & prep fees included. Some dealers quote lowball prices but add back fees for every little thing needed to actually complete the sale paperwork and make delivery. Those can add thousands to the bottom line.