I have been using Google Maps and WAZE during my last couple of long RV trips getting from Denver to Athens, GA in 4 days. If you are traveling along an Interstate highway most anywhere in the US you are going to have cellular data. Momentary loses will not effect your travels or your navigation as many miles of data are already on the phone and the routing is done with the GPS satellites and not a cellular connection.
If you fear you will be in an area with no cellular for an extended period of time you can pre-download the maps covering the area you will be traveling. Loss of cellular will have no effect on your travels. The only thing you will lose is the live traffic data.
FOR ME, it's the traffic data that makes this the only choice for me. Yesterday was a perfect example. I had plans to visit a campground near Chattanooga, TN yesterday. The plan was to take exit 176 and then 1 mile to the campground. My GPS showed taking exit 161. When I looked ahead on the map, I24 was at a stand still for many miles and it I took the planned exit I would be sitting in traffic on the Interstate for an hour to go the next 15 miles. I looked at the road the GPS suggested I take it showed as a nice rural road. So the choice was 15 minutes of back roads or an hour sitting on the Interstate. We took the back road. Ahead of us and behind us were 2 tractor trailers so I knew I had made the right choice. It was a nice easy drive with a 55 mph speed limit (which I didn't do most of the time) and we were still in place and parked before cocktails after a 350 mile drive. I also looked at how much data the phone GPS's use and it is so little as to be dismissed. On a trip of approx. 250 miles Google Maps only used 7Mb and Waze used around 15Mb. This won't have any impact on anyone's data plan.