Really depends on the phone and service provider. Nearly all towns & interstate highways will have cell phone service. I did a tour from South Carolina to Texas last year and had roaming charges in Abilene, TX but always had coverage in every town. The roaming charges got me for 70 cents a minute. Sometimes I would lose coverage in remote areas or have analog service.
Before you go, get a coverage map from providers and base it on the route you will be taking.
Good luck.