Note: Many of those areas without much phone coverage also don't have a dense road network, so turns are quite far apart and not uncommon that you'll follow the same highway all day.
In contrast, the trickiest spots I've found for navigation are in dense urban areas where I am looking for streets/one-ways, restricted roads and opening times for businesses. These also tend to correlate with areas with more phone service.
So what works for me in US and some extended tours abroad including areas without much phone service is a three part approach:
1. For something that gives me a more continuos info about distance/time/temperature - I like having a basic GPS mounted on my handlebars. In the lower 48 it is tough to find gaps more than a day long between services - but in places I've had those gaps I bring an extra battery pack to charge up. I also have some 3x5 cards for cases where I might scribble some route instructions like 35 miles to town and turn right to follow US 87, or service info like convenience stores at 23 miles, 52 miles and 61 miles. I copy that info when I am online and looking over potential routing.
2. For off-line access, I have phone or tablet with MAPS.ME. Typically, I'll pull that up to cross-check location. The phone isn't on continously but instead when I am uncertain and doing a quick check. The phone is in the handlebar bag otherwise.
3. For on-line access, I have an unlocked phone with dual SIMs. Overseas, I'll cycle through SIMs of the countries I pass through and having more than one is useful. In the US, I would suggest looking at some of the resellers and picking two with different networks. For example, go here and pick two using different carriers (
https://en.m.wikipedia.org/wiki/List...work_operators). I've used Net10 but you may get better deals for your needs. With one phone and two SIMs on different networks you can get pretty good service - particularly if you consider the most likely use is areas with dense road choices and likely also phone service.
Now as far as maps go - I wouldn't necessarily buy all the maps from all the routes - but instead consider routes expect to spend considerable time - or with more complex routing, e.g. Pacific Coast route in Southern California crossing several large urban areas. You also don't need to buy all in advance. I have ordered some to be delivered "general delivery" address in upcoming city I knew I would pass through (I generally try to pick a city big enough the Post Office has reasonable hours but not so big that I have to figure out which of many Post offices it gets sent). Aside from Adventure Cycling maps, I also like having an occasional paper state map to lay out and do more general routing.