I've heard good things about Gaia, but I'll throw another name in the hat. I started using Backcountry Navigator this year and love it. I don't know if Gaia has some features that BCN doesn't, but I've been more than happy - it has done everything that I need. Download topos and aerials for offline use, mark way points, etc, etc. And it was only a $11.95 one time fee - no annual renewal.
It works in airplane mode without cell service, and battery life is excellent. One thing that helps is to turn the GPS function on and off on your phone and use it only when needed rather than keeping it on all the time.
Last fall I went 8 days with a single battery pack to recharge my phone. I had along a solar charger but never used it.
I also had my old Garmin gps with me and never turned it on. A cell phone map package has made them obsolete, at least for my purposes.