You can look at a 2005 Itasca Horizon 40D on the online NADA.com website. That will give some clues, but it may be low if its as nice as you say. The onlione NADA is based on average depreciation, so may not reflect market values in your area.
You should also check RV ad sites such as rvt.com and rvclassified.com to get a feel for the asking prices for similar models. Remember, tho, those are asking prices and not necessarily what they would sell for.
You can view some actual selling prices on the PPL motorhomes site. These are consignment sales rather than dealer prices. Possibly lower than a dealer would sell, but close.
Another source is the subscription
NADA or Kelly Blue Book price guides. These are more accurate than the online NADA version because they use market data.. Your local library may subscribe to one or both, and your local banks and credit unions almost surely do.
Used Recreational Vehicles Sold or for Sale - PPL Motor Homes
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL