What type of bag should I use?

I'm traveling in United States. I'm traveling to different cities in 10 days. I'm going to use buses as my source of transportation. But, my problem is that I don't know what type of bag should I use. Because I'm going to jump from one city to another city in just 10 days. I'm sleeping on the bus. I want to bring a bag that could fit all my clothes. And I want it to be easy to bring along. At the same time, I have my Nikon D80 to bring along, my books (just 2 probably) and of course my toiletries. If there's a bag that has a small bag attach to it, so that I could separate them whenever I wish, that would be great.So, that when I can, I can put the big bag in the bag compartment and I can hold on to the small bag (where my books, my camera and my toothbrush going to be). But, when I move, I can just attach the small bag back to the big bag. Any suggestions?

You should to bye a midium size travel bag.