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Did you look at the Kelly RV bluebook at one site? Where else did you look so I don't duplicate what you have already done?
I am attaching a link to a whole list of these that are for sale. Compare them to what you have and perhaps use the NADA guide IF you know all the features yours has. They still are going to use pretty much a low figure.......you just have to argue you point. Let me know if you need more as all follow ups are included in our fee. https://www.bing.com/search?q=1991+airstream+pinnacle+for+sale&form=EDGNTC&qs=AS&cvid=5552e5a51dec46ef8f11b6e3e8b14943&pq=1991%20airstream%20pinnacle%20&PC=TBTS
Perhaps use this one with a lot of photos as a guide...........priced at $12,000. http://www.usedrvsforsale.com/product/used-1991-airstream-rv-pinnacle-30-443627-9
Just checking back in to see if that information proved helpful and if you might need more from me on this?
I think you may be overthinking this...........pick the highest ones that show yours in a good light. There is no one site that gives you all of them. And they can be listed as any of those you mentioned. Same unit. Airstream is a division of Thor and Pinnacle is just the model. If you can get them to give you $13,000 I suspect you have done well, unless you can prove yours was much better that those listed. That is were the Book value might be helpful, buy you have to know exactly every feature and option you have and most don't or forget some. Keep in mind that most of the time they try to lowball you and it is up to you to reject that first or second offer and work them up to one you are happy with. Hope it works out for you!
At the very least it shows them you have done your homework..........and I suspect more than they will have done.