Dealers Profit Margin

For various personal reasons our motorhome is not going to get the use it needs and we have therefore decided to sell. We prefer to go to a dealer and are prepared to 'take a hit' on value but just how much should we accept? We have established that a dealers price would be £35000 and are prepared to accept £30000 but is this a big enough margin? Do dealers operate on a bigger margin? I appreciate that it depends on condition and extras etc., but it has been well maintained, is  well looked after and has many small extras, so we would be very surprised if a dealer had to spend a significant amount to market it.

Any opinions/comments would be appreciated.