Coverage maps are available for the various networks that indicate where reception is or is not available. Consumers Reports has accurate information as well on this topic, as well.

Where specifically do you plan to go? I am not sure which state parks you will be visiting, but many of those I know are often not tremendously isolated, and they are typically rather well developed, to the extent that land lines are typically available. They usually feature friendly cooperative staffs who will come to your aid.

I have used Verizon with great satisfaction for about seven or eight years. I have been able to get out, even from places where reception should not have been available. My understanding is that Verizon has the most comprehensive network to date.

I go well back into the pre-cell phone era, when you jolly well could not communicate at will if things got tight. You just had to deal with the problem. That is still the best strategy.