Grant is 100 percent correct. Just tell them you need insurance and when they ask what you want to insure just say "Motorcycle".
NOw of course in the states once legal in one state its legal to be driven in all. SO I would guess any vehicle with a foriegn license thats allowed to drive here would be legal in all states.
NOw, heres the hard part. There may be separate licenses or fees to ride in certain state parks. I know here in Ohio there is a license reqired to ride in the national forest. Its just a matter of paying the fee. BUt, finding a place when you need one is hard.
SO there may be local license that are needed.
But, from what I see regarding the Trans AMerican trail it should be accesible easily by any street legal vehicle. Its seems to concentrate on public domains to minimize the licenses problems, from what I've seen.