We always carry water and some food in the truck. After all, we drive it when the rig is parked.
I really don't worry about no coverage zones - and I spent half of 2014 in areas with no cell coverage - Big Bend, etc.
I've also stopped when I see someone having trouble, but would almost never carry anyone in my truck to an area with phone service. Might take them to the nearest gas station, store, etc. Most likely not.
Also remember what used to be a universal signal of car trouble. Hood up, white cloth tied on the road side of the vehicle door handle or the antenna.