Agree with RacerX. Google maps is fine on the road and in urban/suburban settings. Off road and distant from cell towers a phone base nav system could leave you lost. Most car GPS like the 4 yr. old Garmin I use will function on battery for several hours.
If you are going to be spending a lot of time far from cell towers consdier purchasing a handheld GPS. Those have maps more appropriate for the off road adventurer.
Have you looked into navigon? Totally worth the money, I use it with my iPad and iPhone both x love the interface and the ability to download the states I think I’ll want to customize the space I need.