When I lived in a bus I traveled to almost 30 states and spent most of my time in rural areas like national forests. Finding a good signal was a lot easier than you might expect. Get the right hardware with an external antenna and high speed coverage is good in most of the country.
Finding a good plan can be tough. If you can get a good T-Mobile signal they have unlimited plans for a reasonable price. AT&T has data plans that are $1/GB. There are grey market resellers that offer unlimited SIMs at an inflated price.