i was all over the place between manchester, nh to portland, me to bath, up the coastal towns, to acadia (bar harbor) a few months ago. i also have sprint and knew i'd face network problems.
my solution was to activate their $5/mo roaming bundle on my account for the time being, and cancel that option when i left maine. i watched my phone switch over to analog just about everywhere north of portland, that wasn't on I-95.
you'll definitely lose web access on analog, and analog eats the battery a bit faster, but at least you'll be able to make calls.