Just so you know.. most of the twin cities area you'd be driving through has band 12 with t-mobile (multiple sources on Google as well as personal experience working at MSP). Band 12 probably wont be deployed anytime soon in rural areas because T-mobile is only using it in urban areas to penetrate buildings with lower frequencies with less line-of-sight areas. What you'd be wanting to look for is T-Mobile refarming their old 2G (EDGE) towers to LTE. Regardless, even with 2G coverage you can still get calls/text/Google Maps to work.
I travel all of North America for work and I'm in a different state every week. I carry a T-Mobile phone as my primary and a Cricket phone (through AT&T) as a backup. I know most people don't have that luxury of carrying two carriers with them, so I keep up to date on this stuff as best I can. People have been getting really mis-informed with this whole T-Mobile Band 12 thing lately.
Another trivial fact.. Minneapolis metro area is one of the best covered metro locations in North America for T-Mobile. Because they bought out VoiceStream, that was one of their top markets. I remember having them when I lived up there years ago!
Anyway, have a safe trip! I'm jealous you're heading to Seattle. Used to live out there as well and I woul do anything to move back! I just got married out there last December.