By the way, $450 seems like a REALLY high price for a spare tire, unless they are also including a rack and other stuff.
You can buy rims and "usable" tires from a junkyard. Just once the tire is more than 7 years old, it is time to replace it, as you don't want the rubber to blow out the first time you use it. Yes there are a lot of delivery dually vans that end up in junkyards, thus spare tires from them.
Current 2005 and later Ford Dually trucks use 17" rims, not the same size as your motorhome. But there are hundreds of dually vans used in shuttle service, school bus, delivery package service too.
Fred.