Booking hotels: kayak, or booking, or Airbnb
Offline maps: CityMaps2Go (preload with data for each state you'll visit, or the major ones)
Food, gas, &c: yelp & google
Preload google maps for one expected route
All these saved me time and trouble multiple times.
Extras:
Attractions and other travel ideas: I'm trying Triposo for this, but I don't have experience with it yet. (CityMaps2Go also minimally lists attractions)
If I plan hiking, I take app hikingproject and preload a state
Entertainment: kindle, audible, any other favorites, audiobooks or lots of podcasts, music, headphones
We have a limited amount of roaming data on our phones, and I only hit this on car trips. It kind of sucks to lose data access in the desert when you're using it to nav... of course paper maps work too as long as you know where you currently are
