Our Verizon CDMA multi-mode phones seem to give us pretty good service where we've tried them, on Silver Service Miami FL - Charleston, S.C. and on Sunset ORL-JAX-NOL. They revert to Analog Roaming when they run out of Verizon's digital system, but the majority of the time we have usable signal, at least on the routes I specified.
I would suggest checking your provider's coverage maps against your proposed routes, and that should give you at least a fair idea of when you might have coverage. And if your provider has a network-update service for their phones (usually a special number like #891 or something that you call with that cellphone), USE IT just before and once or twice during the trip, as that will help educate your phone as to who (what networks) it is allowed to talk to, and how to look for and connect to them.
Make sure you have nationwide roaming and nationwide long distance (not just where you can call long distance TO - also where you can call long distance FROM, without extra charges), and make sure you know exactly what the extra charges, if any, will be for those things, or you may pass out when you get the bill. If you don't read the fine print, you could discover that your "nationwide long distance" is only without extra charge when called while you are in your home territory. Cellphone tariffs can be really tricky, sneaky things to try to decipher.