After my freshman year of college I hitchhiked from Washington DC to San Francisco, zig-zagging across the US. While I did not travel with absolutely nothing (nor did I hitch the entire way; parts of it I went by bus), I wound up spending between $2 and $5 a day on food, so I think my experience is sufficiently similar to what was talked about in the OP to comment.

There were several important points missed by the OP. Hitchhiking across long distances is slow and incredibly discouraging--especially in featureless areas like the US Midwest. Heck knows how people on the Oregon trail managed it. People almost universally insist on dropping you off in a town or a city, but nobody picks up hitchhikers inside of a town or city (unless they're willing to pay their way), so you have to walk back out into the middle of nowhere before you'll have much of a chance of picking up a ride.

If you ever do find yourself hitchhiking, you want to immediately protect yourself from the climate. (This ought to be obvious, but apparently isn't based on my interactions with some people regarding this sort of thing.) In places where it's cold, you want to be sure to be wearing something warm and dry; in places where it's warm, you'll want to have bottles for water, and a means to carry it (and I don't just mean the dinky kind you can usually get as bottled water...by the end of my journey I was happily carrying two 2-liter soda bottles and a 1-liter bottle someone had given me for water in the backpack I had brought with me).

Speaking of items to have with you, a towel actually is useful--something large enough to use as a blanket to cut down on heat loss due to wind, something you can wet down and wrap around your head to cool off in high heat, something to help clean yourself off when the opportunity presents itself.

Whatever you do, you should try to be aware of local laws and practices and how they will affect you; for instance, while it is technically illegal to walk along interstate roads in most of the US, it's not likely you'll attract police attention unless you walk dangerously close to traffic, or appear to be obviously armed and dangerous. The one time I was picked up for this, the cop dropped me off at a nearby exit and gave me directions on how to efficiently complete the next leg of my journey using only state and local roads.

This segues nicely into another general principle. Be nice to people you run into. As long as it doesn't look like you have anything worth taking, essentially every human interaction you have will be positive as long as you act positively towards others--this is more true than usual, since, as a hitchhiker, no one can really expect to get everything from you, so the only people who will interact with you at all will be people who want to help you. Such people can be easily discouraged if you don't appear friendly, or grateful for their assistance.

There's a lot more to be said about the hitchhiking experience, but the most important point to drive home is this: traveling long distances, mostly on foot, is not to be undertaken lightly. It is grueling, unpredictable, and often discouraging. It is not a good way to get from point A to point B, when they are separated by large distances. But it will let you meet a variety of interesting people in--and see a lot more of--the space in between.