I cannot answer the blue book question for you but if you don't mind a little incite, I have a few thoughts.
First off, IMHO, blue book prices are just guidelines. For example, there is a big difference in vehicle values between the three "major" ones.
Not to long ago, I sold my horse trailer via Horse Trailer World. While reviewing that and various other sites, I followed prices all over the country. Like horses, there is a glut of trailers on the market with a lot fewer buyers. I saw comparable trailers to mine go for rock bottom even at dealers. I also observed trailers that had many hits but were not selling. More than likely due to the asking price. Like real estate, location is a big factor.
I would garner up some comps and make a low but somewhat reasonable offer. Be polite about it and briefly state that is what you can afford but don't be offensive or whine. Leave a callback number. That's how I got 2k off the asking price on another trailer awhile back.