Haven't done cross country, since I live quite near the geographic center, but have done a few 3000-5000 mile loops from the center out to the edges. I find that I pack a lot of stuff I never use, and still have have to buy an occasional item that I didn't bring along. If you are traveling in the U.S., what you can get from home, you can get almost everywhere. Almost every place with population 10,000+ has same national chains and local businesses of any other place of the same size.
If there's something I didn't NEED enough to buy one, but would have liked to have, I keep a list, for packing the next trip. I should probably also manage a list of what I carry that I don't use. Everybody's needs are different, we work out what they are based on personal experience.
What's a whole lot harder is packing for a 15-20 day cruise or bus trip, one bag, no laundry, a foreign place you've never been. But I got over worrying about that when my bag got lost in Budapest, had four days in Prague with what I was wearing and what I was carrying in my daypack (documents, money, camera, rain jacket and meds). A few of my fellow travelers were doing the whole trip from a single carry-on. I've also done a week in Yunnan province, by local buses, withwhat I could carry in a backpack. Only difficulty for that one was finding a decent razor in China to deal with a Western beard. So what one really needs to bring for a trip of any length is quite personal, depends on what you need for comfort, what's available where you travel, what you can get locally.
Confession: for 47 of the last 52 years, my wife did most of the lists and packing, and I concerned myself only with preparation of or travel vehicle and my photographic equipment. The past four years since her death have been a learning experience. The what to do before leaving list (hold mail, leave key with neighbor, roll over phone, call security, bar the sliding doors, turn off water, etc) has been trickier than the what to bring list.
Tom Test
Itasca Spirit 29B
