FWIW, just in general, I would say Americans and Canadians lie somewhere in between Germans and Spanish in terms of flexibility in plans and doing things according to protocol. (I lived in Spain for a long time, and just about everywhere you could always find reservations, no matter the situation, after some polite banter. Some exceptions in Catalonia and Euskadi, but very few. The Germans I knew tended to not even consider anything outside of the specified norm, as if even trying was very impolite. Neither is good or bad, just my general observation.)

That said, and as Macha said, a lot will depend on weekend vs. weekday and the area. If you are in Yosemite or Banff on a three day weekend, you might not find a place. Also keep in mind, our tourism here in N. America is not as season specific as in Europe, (though from May-August is somewhat higher, just not the huge spike like in Europe).

Lastly, don't be afraid to ask people. Most N. Americans, contrary to rumors, love to show foreigners and out of towners the best of themselves and their homes. I assume you're on warmshowers, but if not, that's another highly helpful place for touring.