The last that I knew, CarFax did not do RVs, but you can always contact them to ask. On the "NADA facts" I am going to assume that you mean the break-down of price information? If so, the information that is free on their web site is better than noting, but not nearly of as much value as would be the information found in the book that they sell. You can probably find a copy of the current RV issue of either the NADA Guide, or the Kelly Blue Book for RVs in the main library, reference section. Which of the two is more used by dealers varies from one area to the next. Either one will give you the wholesale, high-low & average retail prices as well as the loan value. You can assume that a dealer paid no more than wholesale for any used unit on the lot, whether a trade-in or one he has purchased. I don't know why you would want to show it to the sellers, but it would sure give you some numbers to work from. You will probably find that most used RVs that are for sale by the owner will be over priced. The main reason is that dealer paper on RVs is usually written for too high a value when compared to the actual value of the RV. The RV loan market has a very low default rate so the financial people frequently loan well over what should be loaned and, just like with a car loan, the payments do not keep up with the rate of depreciation, particularly for the first few years. Also, my friends who have sold RVs for a living tell me that most buyers pay more than they should for their RVs and very few even come close to the low price that a dealer will accept. Also, sales people are very good at manipulation of the numbers to make bad deals see to be good.
With a new, current year, RV, you can attempt to get about 20% or even 25% off of MSRP. With 'older' models, treat them as if they were used, because that is what they will be worth the instant you drive them off the lot.