Probable cause is defined very broadly by some LEOs, and even more so in some jurisdictions. Being quiet, polite, and well groomed may not be enough. If an LEO wants to search your pack he or she probably will and there is nothing you can do but politely state that you don't consent. Some LEOs have no problem stretching the law in order to enforce it. That being said, many LEOs that you meet will be a great help to you also, but they still may want to look in that bag first.

I had a friend that hitched and hiked across the country roughly 20 years ago, but he wasn't alone. He had a large, well trained German Shepherd dog with him. He said he didn't have a single problem except for one male driver propositioning him, and that guy was polite when politely turned down. The dog tended to bring out the best in good people and discourage others from bothering him. My friend and the dog were both quiet and kind but neither looked like easy targets either since they were both large for their breeds.
I don't know if he could make the same trip, with the dog that is, today or not.