From the research I've done so far, Google Earth only uses what data is in the cache, so no way to download maps for fully offline mode viewing. Google just needs to combine Earth & Maps for desktop, since they are nearly identical anywayz.
I've seen a program called ExpertGPS, just learned that is supposed to function completely offline. It has a 14 day free trial, $33 annual sub. I never looked at it before as I already have a sub to Gaia, don't want to pay subs for a whole bunch of different programs / apps. I have another 150 mile trip coming up Monday, so will download the ExpertGPS & test it on the trip. Will also see how it compares to Gaia & OnX for off road mapping.
I just learned there are Android emulators available for windows 10 that run Android apps. If ExpertGPS doesn't work for me, I will look into an emulator to run Gaia app on laptop. I created three offline maps in Gaia Android, a northern & southern map that completely cover the area in the adventure map satellite photo I uploaded earlier, and one offline map covering the Great Plains getting me from MI to the west. The Great Plains map seemed to track live offline today with fine, nearly street level view.