I know you're talking about a motorcycle, but a cross-country bicycle ride can be even more liberating, albeit more time consuming. There's a company called Adventure Cycling who run cross-country trips every year (for about 3k-6k, for about 3 months, if my memory serves). Factor in the cost of a touring bike + gear and you're looking at about half the cost. Now that doesn't say that you'd be able to hop on a bicycle and ride cross-country either, but it'd be easy to build up stamina to do so, just takes time, and miles.

That said, you could also apply everything from bike camping websites to motorcycle riding. The equipment is roughly the same, though if you're doing the trip on a motorcycle, you'll need slightly different panniers. Just something to think about ;-)

Sounds like it would be a hella fun trip!