Map caching is something we've been investigating, but haven't yet found a consistent answer.
That said, the workflow that has been described to me is something like this:
1) when connected to wifi, open the map app that is the default for your tablet (google maps app or apple maps app). Or both to be sure!
2) zoom in on the area that you intend to fly and allow maps to load fully.
3) turn off wifi.
Perhaps this is the route you're already taking, but it has been suggested on more than one occasion and will hopefully help. We will continue investigating meanwhile.
Thanks Phillip, hope this helps!