Depending on how old it is you can probably find a selling price for it on the North American Dealers Assoc. website
http://www.nadaguides.com/? ?Click on Recreation Vehicles Top/Center
Bear in mind that NADA prices are higher than Kelly Blue Book. Make sure to list all of the accessories contained in the motorhome as this can significantly effect the value.? You might also lurk on the ebay website under motorhomes to see what comparable vehicles are selling for.
Hope this has helped.
Woody
[edit]NADA URL[/edit]