Check the date code on the tires. That's an expensive replacement no matter how fancy a tire you choose (for six fancy RV tires - Goodyear G947 RSS - I spent $2340 all in). Do a little research, but you'll see lots of discussion about tires aging out after 5-7 years regardless of wear. If the tires are old you can either negotiate for a discount or for new tires.
Are there any other mods you would want done that you could find to trade them? For instance, our unit didn't originally have a ladder and I said I'd accept their "last" price if they installed a ladder.
Are there any others available within a reasonable enough area to use in negotiations? We were willing to travel quite a way to buy ours, so we negotiated when there were five available across the country and used that against the various owners/dealers to get the best deal we could.
As for evidence, have you looked at
PPL Motor Homes? If I recall correctly, they leave a list of their sold units so you can look back and see pricing, etc. Maybe they have the same or a similar rig?
I also agree with the out-the-door price...I do this in all vehicle negotiations. Let them monkey around with dealer fees, taxes, etc. You don't get surprised, you say I'll give you $X out the door, take it or leave it. At least, that's what I like to do. :)