The GPS part of Google maps will work even if you are offline. The problem is that Google maps doesn't keep the entire map of the United States (or whatever country you are in) in your phone, and relies on Wi-Fi to download parts of the map as you travel.
That being said, Google maps does allow you to download the map for your entire route before you start off. So if you are going in and out of an area, you can download the map while you still have Wi-Fi, and use it when you don't have service.
I have not made 10 posts yet, so I can't post the link. But if you Google "Google maps download route" the instructions will come right up for you.
Of course, this does not help if you get lost and need to find your way out, nor does it help if you are exploring. But you should be able to download a large enough part of the map to help you in some of those cases. You just need to think ahead and download a section of the map before you head out, if that makes any sense.