What's not mentioned here is the brand, age and general condition. If its a high end and in good condition overall, then you'll pay more. If the Rv is on the low end on the quality curve and older better give it a low ball bid. To some extent it depends how you'll going to use it.
We bought an Excel from an RV consignment dealer. So we made an offer something like 30% below asking price and the owner accepted, she was motivated to sell. The dealer swapped out some cheap tires for GY G614s for about 30% of the retail value of the new tires.
I agree with the advice of researching RV Traders and the like, That's exactly what we did prior to buying. Stay away from Craig's List unless you understand the RV scams there.