Traveling with a baby within the United States is easier than a lot of places in the world due to the sheer amount of services available in any given town or city. You can stop pretty much anywhere and get what you need.
That being said, there are still some things that you don't want to be caught without when you're traveling with a baby. Some are just inconveniences, others can put your baby in danger. Here's a short list of what you'll need if you're to go traveling with your little one.
First thing's first: diapers and wipes. Even if you're cloth diapering, you're going to want to go with disposables on road trips simply due to the lack of easy access to laundry services. Who wants to ride around with a poopy cloth diaper stinking up the car for the next 150 miles?
While you're at it, pack some hand sanitizer, as well. If you've just handled wet or poopy diapers, you'll probably want something to clean your hands with.
Next is food. You'll obviously have to bring formula and some baby food, but having an insulated cooler to keep it in is a good idea. Diaper bags may have insulated pockets, but they may not be quite big enough if you're going to be on the road for long hours and don't want to have to stop to make bottles.
Toys and some kind of entertainment is also important. Even babies as young as 3 months can benefit from having a tablet that they can watch cartoons on in the back seat while you're driving. It's hard to keep a baby sufficiently stimulated to prevent them from throwing a fit, so the more entertainment options you have for them, the better off you'll be.
A collapsible stroller and other travel related products like the ones on House Mama could make a huge different to you and your baby. You'll eventually have to get your baby out of the car and walk around. If they can walk, let them. If not, riding them around in a stroller so they can see new things is a little entertainment for them and a break from riding in the car seat.
If you're going to have a couple of stays in hotel rooms, you'll likely want a travel bath tub if your baby is and infant or under 6 to 8 months old. It makes a world of difference. There are several of these on the market that range from collapsible to inflatable to save space and weight. You can find these in list post also like this one on House Mama. Just a few quick searches and you can have untold options that are made for very specific needs.