The short answer is DON'T DO IT!
We have an unlocked smart phone and bought a SIM and a 3 month snowbird plan from them, started the trip down here in November. We transited the US from Detroit to southern California.
We had virtually no high speed data, mostly had "E" (edge) which is slower than dial up, or no service. In terms of phone service we had a number of places with no service, we also carry a cheapie TracFone which had good coverage and used the Tracfone when the smart phone had no service.
AT&T or Verizon may be a better choice. I understand that Cricket was taken over by AT&T, they may also be a good alternative if you have a GSM phone, bad news is these companies are probably more expensive.