You could have just gotten a GPS app like CoPilot where you can download the entire United States at once.
Functions just like any other GPS and you don't need a cell signal at all to use it.
I swear by CoPilot GPS on my iPhone. It downloads the entire map to your device (no guessing which part of the map you need, or what resolution). No data connection needed when you are on the road. I have so far used it in Ireland, Germany, Utah, Arizona, upstate New York and New England USA.
I absolutely would not trust that you will get a reliable mobile signal on the road, especially not around mountain regions. Getting lost in a foreign land is not something I want to do.
OsMAND is a free alternative for Android. I haven't used it for navigation since the iOS version doesn't support navigation.
CoPilot GPS is our goto. It includes propane restricted roadways as well as customizable height and width information. Some other users reported it requires a ton of local storage for all the maps (we only use the limited areas we drive in).
I think your best bet is to start your research and decide what destinations you want to visit.
Once you do that, then go on some of the Almanac type sites that have historic weather reports
As you can well imagine, you want to save the northern most regions until after the thaw out. (April-July most places are thawed and safe depending on how far north)
The deep South can get pretty hot and humid in the summer months, so you may want to be in Canada in July, etc. so check historic weather for the cities, find what is the best time to visit them (or just do a google search for best time to visit_____ and plug in your destination).
Also, you might want to get an app on your phone called "co-pilot." I love that app. You have to pay for it, but it can tell you if there are any train trestles, or over passes on your route that you can't fit under in your RV (it happens).
And then cross your fingers. No excursion that long will go off without a hitch, but you can give yourself a good chance to succeed with good pre-planning.
It should be noted that Google Maps doesn't have offline bike routing (even in countries where it is supported online), but it does have offline maps for traditional navigation. However, I agree with others, there's no need to use just one map application. I've got three on my phone regularly. Google Maps, Backcountry Navigator and CoPilot GPS (and CoPilot does do bike routing)
I use CoPilot.
It is an offline navigation program, and for the US, it only costs $9.99, and comes with free map updates.
Their website has links to the appstores of the three major mobile OSs.
You can load a few states or even the entire US (2GB, and it can be put on your SD card)
I can't recommend it enough. You don't have to figure out how to pre-download an area, just chose the state or country you want downloaded.
Lots of people use RV trip wizard. But you have to prep lab everything in a computer first and then you can access them in an app. The app is CarPlay enabled which is great. I forget what it costs.
I used https://copilotgps.com/en-us/ last year $60 and it works through the app.
My parents sold their full timing rig last year and gave me their RV specific Garmin. So I am going to try that next year.
CoPilot GPS app will let you download a country or region map. Once you do that, you can use it anywhere without an internet connection.
The cost is about US$30, which sounds high but realize you'd pay almost as much for paper atlases. I bought it years ago, rarely use it anymore, but it's nice to have just in case I can't get a GPS signal.
We drove in Ireland a few years ago. We did find that a lot of the countryside has zero signal (T-Mobile), so we relied on an app which allowed me to download the entire Ireland map.
The app I used was CoPilot GPS. It costs money, yes, but not much more than a paper atlas would cost, and you get free map updates for life. Well worth it.
EDIT: I see you use maps.me. I don't see any reason why it wouldn't work in Ireland.
Many years ago I paid like $30 for CoPilot GPS. I know you specified "Free", but I have access to updates of all the worldwide maps, and I never have to worry about seeing ads or whatnot. It costs less than a single printed atlas would have cost in the old days. It's always there on my phone. I've had it for years, over 3 different phones. It's worth it.
Sometimes, just traveling in rural areas in the US, I just find I don't have a signal, and I know I can always count on having CoPilot there as a backup to Google Maps.
https://copilotgps.com/en-us/compare-features/#pricing
Note that the voice navigation is optional; it's $26.99 for the fully functional app, no hidden charges.
For $30 you can get an app like CoPilot GPS for the phone. Just download the entire map of Scotland (before the trip) and Bob's your uncle.
Then get a QuadLock case for the phone and car mount.
It's an app for iPhone and Android, not a standalone GPS unit, but CoPilot GPS will give you audible alarms if you go over the speed limit:
https://copilotgps.com/en-ca/speed-limits/
You can set it to warn when exceeding 5mph, 10, 15, 20, 25, or 30 mph over the speed limit (I'm sure there are similar options for kilometers). It can give visual warning or audible warning, or both.
I also use Motion-X (mostly for hiking). Motion-X can download map data piecemeal, go to Menu/Setup/Map Downloads, and then you select a radius to download. I prefer CoPilot, because it has turn-by-turn navigation, and because CoPilot lets you download the entire country ahead of time.
I have used CoPilot in Germany and Ireland, (and all around the US) and I plan to use it in the Netherlands and Belgium this summer. It downloads the entire country map (do this at home before your trip!), so it can work offline. CoPilot has a listing of attractions, but it is not as complete as Google.
I like CoPilot GPS. I've used it many times in areas where I had no data signal at all (thanks, T-Mobile!). I've used it in Germany and Ireland. I have the ActiveTraffic subscription, I like the way it shows you traffic problems along your route.
Try CoPilot GPS. You download the map in advance, before your trip. Then it will work with zero data connection. I have used it with great success in Germany and Ireland, as well as many places in the US.
I use, and recommend, CoPilot GPS.
I have used it in remote parts of upstate NY and New England with zero cell signal (I have T-Mobile, which has terrible coverage), as well as remote Utah, Arizona, and internationally in Ireland and Germany.
You can plan mult-stop routes, it has a database of "points of interest" (which is good in some areas, not so great in others; certainly not as good as Google Maps).
You can drive to a point of interest; an address you enter; you can point to the map where you want to go; you can choose any Wikipedia article location.
It has biking, driving, and walking modes.
You can download regions of the US or the whole country at once.
It has free traffic for 12 months, then it's 10 bucks a year after that for the traffic.
You can set it to warn you if you are speeding, if you like.
Let me know if you have any specific questions.