Getting "book value" is tricky as it never seems clear what items should be added or are considered standard on that unit. I don't add for stuff that you would expect to be on the unit when delivered to a dealer but just truly extras such as a 2nd battery or added awnings, etc. You're unit is old enough that probably all you need or can get is a rough estimate and an extra item or 2 isn't going to make much difference. Besides there is no "true value" but just what a buyer will pay. So you can use just about any book value that shows up then add some and be ready to negotiate. The darn things depreciate like crazy.