NADA has RV values, available in their Used Car(rv) Guide book, you might have to go to a dealer to see one- the web site goes back to 1986, so you can get a general idea there. I would suggest knock off 1000-1500 for the year model, but the added equipment (like banks power pack, exhaust brake, gear vendors transmission,etc.) can add much to the value. Also mileage is a big factor in value for the book. Check this site-
Also the RVTRADERONLINE site can run average values for those in their data base.
Third idea- some of the guys here that own the same model could give you a general idea if you have an ACCURATE description of condition. I have seen ranges from 10,000- 39,000 for the 345, all dependent on condition, and market awareness by the seller.Most in the 22000 and up range if nice.