I was, until year and a half ago, a cross country truck driver, and I did exactly what you are looking to do. I used Verizon Wireless for my internet service. You can either get a card for your laptop or get a kit that plugs your phone into the laptop and uses that for it's service. Both are fine for internet. But the card isn't cheap and you will have an early termination fee if you cancel it within a year or two, depending on the contract. The service to plug in your cell phone to the laptop is only an added feature and cancelable at any time. Remember, this is Verizon, so I do not have info on the other carriers. I went coast to coast and you will not have service everywhere, escpecially very remote places like West Texas, but for the most part, if you are on the interstate, you will have service better than 85 or 90% of the time, in my experience. Also, this works regardless of wether or not you are moving. For those times when you do not have service, if you are going to be staying in hotels, you can pick up rechargable dial up kits at any truck stop. You just install it on your laptop and it will us the phone line in the hotel to dial up a 800 number to get you access. it will then look for a local number in the area you are in. If there is a local number, you will get about 3 times the minutes or uses for the same amount of money as the 800. Basically, if you get 600 minutes for $20 on the 800# you will get about 1800 minutes on the local number for the same $20.
Remember that whichever service you use for the internet you will either need the proper card for the laptop you are buying or the proper phone that has an available mobile office kit to hook it up to the laptop.