Got T-Mobile last year just before a trip from Dallas to Rapid City because we needed some new phones and they had a great deal. Our route took us to Raton, NM, then up to northern Colorado (Poudre River area), then through Wyoming to Rapid. After visiting family there, we went to Minnesota and then back home through Iowa, Missouri, and Arkansas. By far the worst coverage was in Colorado/Wyoming. Seemed non existent for hours at a time. My boys struggled because it would jump from third party to third party; many times not having any data - just calling ability.
Check out the coverage maps on their website and zoom in on areas you travel to see if service is there - their maps really are pretty accurate, so you're able to compare companies.
2017 Ford F-150 3.5L V6 Ecoboost
2007 Rockwood 8318SS
1 beautiful wife, 3 camping kids, 1 camping shih-tzu